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 (I(Ca)) with T-type kinetics and voltage dependence. GC in the everted-follicle culture express an outward potassium current (I(K)) kinetics, which remains unchanged during three days in culture. I(K) 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 (I(nK)), which progressively dominates as cells advance from day one to day three in culture. A similar I(nK) was recorded in large luteal cells. A possible link between luteinization and the appearance of I(nK) is hypothesized.[...]