Addition of GABA to the bathing medium in concentrations of 10(-6) to 10(-5) M abolished the spontaneous bioelectric activity of rat Purkinje cells in vitro. Lower concentrations of GABA (less than or equal to 10(-6) M) influenced the firing pattern of spontaneous discharges in such a way that the relative number of long interspike intervals increased. The inhibitory effect of GABA was reversible; the cells resumed normal activity after being transferred to a normal balanced salt solution. The action of GABA was antagonized by bicuculline and picrotoxin, whereas strychnine was unable to overcome the inhibition induced by GABA. Both bicuculline (10(-10) to 10(-6) M) and picrotoxin (10(-7) to 10(-4) M) exerted an excitatory effect on cultured Purkinje cells when added to the medium. This excitatory action suggests the existence of inhibitory synapses in cultures of rat cerebellum in which endogenous GABA is used as a transmitter. The ratio between the dose of picrotoxin and that of bicuculline necessary to produce maximal excitation was in the order of 10(3). Both substances showed an inhibitory action at high concentrations.