Upon fertilization the hamster egg shows transient, periodic hyperpolarizing responses (h.r.s) due to a Ca-activated K conductance; these are superimposed on a gradual, hyperpolarizing shift of the resting potential (h.s.) (Miyazaki & Igusa, 1981a, 1982a). The h.r.s and h.s. were further analysed by changing external divalent cations or by injection of Ca2+ into the egg, to study the mechanisms of the increase in the intracellular Ca2+ concentration ([Ca2+]i). The series of h.r.s was abolished by the removal of external Ca2+. The frequency of the h.r. was decreased by lowering the [Ca2+]o or by adding Mn2+ or Co2+, and it was increased by raising the [Ca2+]o in a time- and concentration-dependent manner. The h.r. frequency was decreased on sustained depolarization with steady current, and increased on hyperpolarization. In contrast to the h.r. frequency, the amplitude, conductance increase and reversal potential of each h.r. were little affected by [Ca2+]o, Mn2+ or Co2+. The h.s. was decreased by lowering the [Ca2+]o, by adding Mn2+ or Co2+, or by injection of EGTA. The h.s. may reflect continuous Ca influx stimulating a Ca-activated K conductance (GK). In unfertilized eggs a regenerative h.r. was induced by Ca injection with an apparent threshold. The relationship between GK and the injected Ca2+ showed a steep jump at the critical current, associated with a four-fold increase in GK. The regenerative h.r. was followed by a refractory period of 1-2 min. In inseminated eggs the periodic sperm-mediated h.r.s. (s.-h.r.s) were interrupted by interposed h.r.(s) induced by Ca injection(s): the periodicity of s.-h.r.s was reset by Ca-induced h.r. In inseminated eggs the regenerative h.r. was induced by Ca injection with a much smaller pulse than necessary in unfertilized eggs. The refractory period was shortened to 40-50 sec, comparable to the period of s.-h.r.s. In inseminated eggs periodic h.r.s similar to s.-h.r.s were produced by continuous, repetitive injections of Ca2+ with constant pulses. The frequency of these h.r.s was dependent on the injection current. It is concluded that each h.r. indicates an enhancement of the increase in [Ca2+]i, probably the result of Ca-induced Ca release from intracellular stores. A possible mechanism for periodic increase in [Ca2+]i reflected in s.-h.r.s is proposed, based on a linkage of the continuous Ca influx to Ca release.