<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      
                                                                                                                  March 31,
                                                1992          1993         1994         1995          1996        1997
                                                ----          ----         ----         ----          ----        ----
                                                              (Thousands of Dollars Except Ratios)
<S>                                           <C>          <C>           <C>          <C>          <C>          <C>
Net income for the period                     $302,748     $297,160      $320,757     $314,107     $304,876     $296,366
   Add:
      Taxes based on income                    197,009      182,716       203,827      207,734      196,210      189,991
      Fixed charges (see below)                136,227      130,914       142,411      138,071      136,102      137,683
                                               -------     --------      --------     --------     --------     --------

Earnings available for fixed charges
   and preferred stock dividend
   requirements of Registrant                 $635,984     $610,790      $666,995     $659,912     $637,188     $624,040
                                              ========     ========      ========     ========     ========     ========

Fixed charges:
   Interest on debt                           $125,798     $124,430      $135,608     $129,239     $128,375     $130,156
   Amortization of premium and discount,
      less expense, on debt; and
      bond defeasance cost                       9,521        5,170         5,504        5,502        4,269        3,811
   Rentals (see note)                              908        1,314         1,299        3,330        3,458        3,716
                                                ------   ----------      --------     --------     --------     --------
      Total fixed charges                     $136,227     $130,914      $142,411     $138,071     $136,102     $137,683

Preferred stock dividend requirements
   of Registrant* (Adjusted for income
   tax effect)                                  21,852       21,537        20,514       20,808       20,612       18,759
                                              --------     --------      --------     --------     --------     --------


Total fixed charges and preferred
   stock dividend requirements                $158,079     $152,451      $162,925     $158,879     $156,714     $156,442
                                              ========     ========      ========     ========     ========     ========


Ratio of earnings to fixed charges
   and preferred dividends                        4.02         4.01          4.09         4.15         4.06         3.98
                                                  ====         ====          ====         ====         ====         ====
</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        
                                                                                                                 March 31,
                                                1992          1993         1994         1995          1996        1997
                                                ----          ----         ----         ----          ----        ----
                                                              (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        $14,058      $14,087       $13,252      $13,250      $13,249      $12,141

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

Non-deductible preferred stock
   dividends                                   $11,973      $12,114       $11,436      $11,434      $11,433      $10,325
                                               =======      =======       =======      =======      =======      =======

Excess of net income before income
   taxes over net income (percentage)
   See note below                                65.1%        61.5%         63.5%        66.1%        64.4%        64.1%
                                                 -----        -----         -----        -----        -----        -----

Income tax effect on non-deductible
   preferred stock dividends*                   $7,794       $7,450        $7,262       $7,558       $7,363       $6,618
Add:
   Deductible preferred stock
      dividends (above)                          2,085        1,973         1,816        1,816        1,816        1,816
   Non-deductible preferred stock
      dividends (above)                         11,973       12,114        11,436       11,434       11,433       10,325
                                                ------       ------        ------       ------       ------       ------

Preferred stock dividend requirements
   of Registrant. (Adjusted for income
   tax effect)                                 $21,852      $21,537       $20,514      $20,808      $20,612      $18,759
                                               =======      =======       =======      =======      =======      =======

Note:  Calculated as follows -
   Net income before income taxes             $499,757     $479,876      $524,584     $521,841     $501,086     $486,357
   Less net income                             302,748      297,160       320,757      314,107      304,876      296,366
                                              --------     --------      --------     --------     --------     --------
   Excess - Taxed based on income             $197,009     $182,716      $203,827     $207,734     $196,210     $189,991
                                              ========     ========      ========     ========     ========     ========
   - Percentage of net income                    65.1%        61.5%         63.5%        66.1%        64.4%        64.1%
                                                 =====        =====         =====        =====        =====        =====
</TABLE>


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