The outward currents elicited in hamster eggs by depolarizing pulses were studied. The currents appeared to comprise at least two components, a transient outward component (Ito) and a steady-state outward component (I∞). Ito was transiently followed by the cessation of inward Ca2⁢ current (ICa), and its current-voltage (I-V) relation was a mirror image of that of ICa. Either blockade of ICa by Co2⁢ or replacement of Ca2⁢ with Sr2⁢ abolished Ito without change in I∞. Intracellular EGTA (10 mM) inhibited Ito but not I∞. suggesting strongly that generation of Ito requires intracellular Ca2⁢. Apamin (1 nM) abolished selectively Ito, indicating that Ito is Ca2⁢-dependent K⁢ current. On the other hand, I∞ was Ca2⁢- independent. Both Ito and I∞ were completely inhibited by internal Cs⁢ and external TEA. The estimated reversal potential of Ito was close to the theoretical EK. Taken together, both outward currents were carried by K⁢ channels. From these results, Ito is likely to be a current responsible for the hyperpolarizing responses seen in hamster eggs at fertilization.