Inactivation properties of Ca current in the unfertilized eggs of mouse were studied by using the whole cell voltage clamp technique and single microelectrode voltage clamp technique. Membrane potential was held at -80 mV and step depolarization was applied from -50 mV to 50 mV for 200 ~ 500 ms. Peak of inward Ca currents was -2 ~ -4 nA at a membrane Potentials from -20 mV to 0 mV and outward currents were not observed within the membrane voltage range studied (-50 ~ 50 mV). Inward currents were fully inactivated within 200 ms after the onset of step depolarization. As the membrane became depolarized, time constant of inactivation (τ) was decreased but remained around 20 ~ 30 ms beyond 10 mV. When Ca²⁺ was used as a charge earlier, inactivation of inward Ca²⁺ current also occured and time course of inactivation was similar to that of Ca²⁺ currents as charge carrier. In the bathing solution containing high potassium (131 mM K⁺), process of inactivation was not changed except a parallel decrease of value for the entire range of membrane potential. Steady-state inactivation of the current(h_∞) obtained from the double pulse experiment showed the voltage-dependent change. These results suggested that inactivation of Ca currents in the unfertilized eggs of mouse was voltage-dependent.