<PAGE>   1

                                                                   EXHIBIT 12(b)

                             UNION ELECTRIC COMPANY
               COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES
                                      AND
                     PREFERRED STOCK DIVIDEND REQUIREMENTS

<TABLE>
<CAPTION>
                                                                                                                          
                                                                    Year Ended December 31,                      12 Months
                                                -------------------------------------------------------------     Ended   
                                                                                                                September 30,
                                                1991          1992         1993         1994          1995        1996
                                                ----          ----         ----         ----          ----        ----
                                                              (Thousands of Dollars Except Ratios)
<S>                                           <C>          <C>           <C>          <C>          <C>          <C>
Net income for the period                     $321,512     $302,748      $297,160     $320,757     $314,107     $316,294
   Add:
      Taxes based on income                    218,954      197,009       182,716      203,827      207,734      210,972
      Fixed charges (see below)                168,380      136,227       130,914      142,411      138,071      136,862
                                               -------     --------      --------     --------     --------     --------

Earnings available for fixed charges
   and preferred stock dividend
   requirements of Registrant                 $708,846     $635,984      $610,790     $666,995     $659,912     $664,128
                                              ========     ========      ========     ========     ========     ========

Fixed charges:
   Interest on debt                           $163,061     $125,798      $124,430     $135,608     $129,239     $128,828
   Amortization of premium and discount,
      less expense, on debt; and
      bond defeasance cost                       4,148        9,521         5,170        5,504        5,502        4,731
   Rentals (see note)                            1,171          908         1,314        1,299        3,330        3,303
                                              --------     --------      --------     --------     --------     --------
      Total fixed charges                     $168,380     $136,227      $130,914     $142,411     $138,071     $136,862

Preferred stock dividend requirements
   of Registrant* (Adjusted for income
   tax effect)                                  22,213       21,852        21,537       20,514       20,808       20,875
                                              --------     --------      --------     --------     --------     --------


Total fixed charges and preferred
   stock dividend requirements                $190,593     $158,079      $152,451     $162,925     $158,879     $157,737
                                              ========     ========      ========     ========     ========     ========


Ratio of earnings to fixed charges
   and preferred dividends                        3.72         4.02          4.01         4.09         4.15         4.21
                                                  ====         ====          ====         ====         ====         ====
</TABLE>


Note:  Represents the interest factor applicable to rentals.
*  See following page for supporting computation.
<PAGE>   2

                                                                   EXHIBIT 12(b)
                                                                     (continued)


                             UNION ELECTRIC COMPANY
               COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES
                                      AND
                     PREFERRED STOCK DIVIDEND REQUIREMENTS

<TABLE>
<CAPTION>
                                                                                                                          
                                                                    Year Ended December 31,                      12 Months
                                                -------------------------------------------------------------     Ended   
                                                                                                                September 30,
                                                1991          1992         1993         1994          1995        1996
                                                ----          ----         ----         ----          ----        ----
                                                              (Thousands of Dollars Except Ratios)
<S>                                           <C>          <C>           <C>          <C>          <C>          <C>
Computation of preferred stock
   dividend requirements of Registrant,
   adjusted for income tax effect*
      Preferred stock dividend
         requirements of Registrant, as
         shown on statement of earnings        $14,059      $14,058       $14,087      $13,252      $13,250      $13,249

Less deductible preferred stock
   dividends**                                   2,085        2,085         1,973        1,816        1,816        1,816
                                               -------      -------       -------      -------      -------      -------

Non-deductible preferred stock
   dividends                                   $11,974      $11,973       $12,114      $11,436      $11,434      $11,433
                                               =======      =======       =======      =======      =======      =======

Excess of net income before income
   taxes over net income (percentage)
   See note below                                68.1%        65.1%         61.5%        63.5%        66.1%        66.7%
                                                 -----        -----         -----        -----        -----        -----

Income tax effect on non-deductible
   preferred stock dividends*                   $8,154       $7,794        $7,450       $7,262       $7,558       $7,626
Add:
   Deductible preferred stock
      dividends (above)                          2,085        2,085         1,973        1,816        1,816        1,816
   Non-deductible preferred stock
      dividends (above)                         11,974       11,973        12,114       11,436       11,434       11,433
                                                ------       ------        ------       ------       ------       ------

Preferred stock dividend requirements
   of Registrant. (Adjusted for income
   tax effect)                                 $22,213      $21,852       $21,537      $20,514      $20,808      $20,875
                                               =======      =======       =======      =======      =======      =======

Note:  Calculated as follows -
   Net income before income taxes             $540,466     $499,757      $479,876     $524,584     $521,841     $527,266
   Less net income                             321,512      302,748       297,160      320,757      314,107      316,294
                                              --------     --------      --------     --------     --------     --------
   Excess - Taxed based on income             $218,954     $197,009      $182,716     $203,827     $207,734     $210,972
                                              ========     ========      ========     ========     ========     ========
   - Percentage of net income                    68.1%        65.1%         61.5%        63.5%        66.1%        66.7%
                                                 =====        =====         =====        =====        =====        =====
</TABLE>


*  Income tax adjustment to reflect pretax earnings required to meet preferred
** stock dividend.  Dividends deductible on federal income tax return.