We studied membrane currents in granulosa cells (GC), immediately after collection or after variable culture time in the everted-follicle wall or in the monolayer. GC in both systems express an inward calcium current (ICa) with T-type kinetics and voltage dependence. GC in the everted-follicle culture express an outward potassium current (IK) kinetics, which remains unchanged during three days in culture. IK has delayed-rectifier kinetics, but is insensitive to TEA, 4-AP and apamine. GC in monolayer culture develop a new, inactivating delayed-rectifier potassium current (InK), which progressively dominates as cells advance from day one to day three in culture. A similar InK was recorded in large luteal cells. A possible link between luteinization and the appearance of InK is hypothesized.