Cerebellar Purkinje cells (PC) can be maintained in culture for one to two weeks in L15, a rich medium known to allow expression of a normal excitability in peripheral neurones. When examined using whole cell recordings, PC proved to be inexcitable in these conditions, and this inexcitability could be related to the presence of large outward K currents. Depolarizing steps of -100 mV revealed a voltage-dependent biphasic K current with a large early transient phase followed by a small plateau phase. The early transient phase could be selectively eliminated by holding the cell at -40 mV or by extracellularly applying 5 mM 4-aminopyridine (4-AP), whereas the plateau was abolished by 15 mM tetraethylammonium (TEA). Hereafter, these currents will be identified as the IA and the delayed current respectively, IA being the predominant current. IA activated between -25 and +65 mV with a midpoint at +3 mV; inactivation occurred between -70 and -20 mV with a midpoint at -57 mV. Current decay followed an exponential time course with a time constant of about 30 ms between -20 and +10 mV. In the cell-attached recording configuration, depolarization elicited openings of two types of K channels, one inactivating and one non-inactivating. The non-inactivating K channel probably corresponded to the delayed K current and had a conductance of 22 pS in a physiological K gradient.