The present study was performed to observe the effects of cations on resting membrane potential and pump activity in the unfertilized eggs of ICR strain mice. After an induction of superovulation, the fresh eggs with zona pellucida were collected and the membrane potentials were recorded. Recordings of membrane potential in this study was obtained from the physiological conditions (37℃ and 4mM Ca in standard solution), differently from the another reports with unphysiological conditions (room temprature and high Ca in standard solution) for a stable and long-lasting observations. Presented data was obtained within 6 hours after collection from the oviduct. The results observed are as follows, 1) Resting potential of the unfertilized eggs was -25.8±3.8mV (Mean±Se, n=31). 2) As the K ion concentration was increased, resting membrane potential was depolarized but showed hyperpolarization with K⁺ below 25mM. 3) Alteration of the resting membrane potential for the changes of Na⁺ concentration were hardly observed, while resting potential was hyperpolarized as Ca²⁺ concentration was increased. 4) Pump activity as transient or prolonged hyperpolarization was -2.29±0.75mV (Mean±Se, n=16), the hyperpolarization was increased in both amplitude and duration under the 10mM Ca²⁺ solution. 5) Hyperpolarization due to pump activity was decreased or disappeared by 5 X 10^-5 M ouabain treatment and could not be observed under the both Na-free and Ca-free solutions. 6) Above results are likely to suggest that the resting potential of the mouse unfertilized eggs is affected to mainly by Ca-dependent K conductance and Na-Ca exchange mechanism and that there is pump activity coupling between K⁺, Na⁺ and Ca²⁺.