The electrical properties of the membrane of the ovarian oocyte at the germinal vesicle (GV) stage and of the ovulated egg of the mouse have been studied using a two-microelectrode voltage-clamp technique. The stable resting potential measured with a single electrode was -38.2 +/- 2.8 mV SE (18 oocytes, 5 animals) and -27.8 +/- 1.4 mV SE (28 eggs, 8 animals) in a solution containing 20 mM [Ca2+]0. The lower values appear to be strongly affected by damage due to electrode insertion. However, there was no evidence of the resting potential being more negative than -40 to -50 mV. Voltage-dependent inward current could not be activated from a holding potential (Vh) close to the resting potential. When Vh was set at -90 mV, depolarizing pulses activated a transient inward current in both oocytes and eggs. The threshold voltage, peak voltage and inactivation vs potential curve were very similar in oocytes and eggs. On the other hand, the current amplitude appeared reduced in ovulated eggs, whilst times to peak and inactivation time constants in eggs were significantly longer than in oocytes. In oocytes the inward current was blocked by 10 mM Co2+ and decreased by lowering [Ca2+]0 to 5 mM similarly to the results reported for eggs. It therefore appears that GV ovarian oocytes possess Ca2+ channels which differ from those present in eggs mainly with respect to their kinetic properties. The physiological role of this inward current remains obscure in both preparations since they are almost completely inactivated at the resting potential.