In this work we studied the excitable properties of cerebellar granule cells grown in primary culture in the presence of "high" KCl (25 mM). Whole cell patch-clamp records of currents were obtained from cells at 1-33 days in culture (DIC). Sodium currents, blocked by TTX, were present from the first DIC and did show slow developmental changes. Two types of potassium currents were detected at all DIC: a delayed rectifier current (IK) and an inactivating current (IA). Both IK and IA increased until 7 to 9 DIC (four and two times respectively). Most of the IA increase, however, correlated with an increase in cell size, monitored by measurements of cell membrane capacitance (Cm) and the current density thus did not change. Conversely, delayed rectifier potassium current density did increase in the initial DIC (3-6) and did not change significantly after this time. Calcium currents were not detectable at any DIC under our experimental conditions.