Whole cell patch-clamp techniques were applied to cultured smooth muscle cells isolated from the longitudinal layer of the late pregnant rat myometrium. Effects of estrogens on Ca channels were examined. Inhibitory effects of beta-estradiol (1 microM) on Ca channel currents were recognized. The inhibitory effects of beta-estradiol depended on holding potentials. beta-Estradiol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 9). Diethylstilbestrol, a synthetic estrogen, gave similar effects on Ca channel currents at lower concentration (2 microM) to those of beta-estradiol. Strong inhibitory effects on Ca channel currents were obtained by higher concentration (20 microM). Diethylstilbestrol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 5). The results indicate that estrogens influence the voltage dependency and the whole cell conductance of Ca channels of pregnant rat myometrial cells. The acute effect of estrogens may cause both electrical and mechanical depression of myometrium.