Studies were done to investigate the transepithelial current-voltage (IT-VT) relationships of urinary bladder and colon of the toad Bufo marinus. Like several other Na transporting epithelia, the IT-VT plots characteristically showed a break at voltage E1, averaging near 124 mV for urinary bladder and 110 mV for colon. With bladders treated with antidiuretic hormone, estimates of ENa and shunt resistance, Rs, were obtained according to a method outlined by Yonath and Civan, 1971 (J Membr. Biol. 5:336-385). Our results not only confirmed their observations, but were consistent with the notion that the values of E1 (IT-VT plots) were the same as those of ENa. In addition, the values of Rs were found to be the same as those estimated from the quotient E1/I1 obtained from the voltage and current coordinates at the break of the IT-VT plot of bladders studied in both stretched and unstretched states. Amiloride at concentrations up to 10(-5) M caused a small decrease of both E1 and  E1/I1 of urinary bladder. Similarly, amiloride caused small but significant changes of ENa and RNa of the colon. For both epithelia, the values of E1 and E1/I1 of the IT-VT plots were the same as those of ENa and Rs estimated by an independent method. In general, these findings are similar to those of several other epithelia where the ENa and Rs can be estimated directly from their IT-VT relationships.