To examine the electrophysiological properties of luteal cells and the relationship between membrane potential and luteal steroidogenic capacity, the membrane potential of luteal cells and the luteal steroidogenesis were measured under different ionic conditions following treatment with various drugs and gonadotropins. The membrane potential of luteal cells did not vary throughout the estrous cycle and was -55 ± 1 mV. The membrane potential was highly dependent upon the external K⁺ concentration and was depolarized by the deprivation of external Ca²⁺, however) there seemed to be a lower K⁺ permeability in luteal membranes as the presence of 10^-9 M valinomycin, a K⁺ ionophore Caused hyperpolarization from -55 to -91 mV. Luteal progestin production was increased in a high K⁺ solution but not m a Ca²⁺-free solution indicating that Ca²⁺ may be essential for steroid synthesis and/or secretion by luteal cells. Gonadotropins and ouabain induced a depolarization of the membrane potential and stimulated luteal steroidogenesis; however; prostaglandin F₂α stimulated only steroidogenesis without any changes in membrane potential. These results suggest that the relationship between steroidogenesis and the changes in membrane potential by drugs and gonadotropins is still obscure and remains to be eluridated. The relationship between membrane potential and steroidogenesis in the luteal cell may be dependent upon the availability of intracelluar Ca²⁺.