The effect of an anticonvulsant compound (Simiand, Ferrandes, Lacolle and Eymard, 1979), 1-methyl cyclohexane carboxylic acid (CCA), upon the electrical activity of Purkinje cells (PCs) was studied in the cerebellar cortex of the rat in vivo. Cyclohexane carboxylic acid (200-400 mg/kg i.v.) decreased the spontaneous simple spike (SS) activity of the Purkinje cells tested without modifying the complex spike (CS) frequency. Two effects of CCA upon intracortical inhibition were observed: (1) the decrease in firing rate that followed surface stimulation of the parallel fibres (LOC stimulation) was enhanced after injection of CCA; (2) the depression of the antidromic field potential of Purkinje cells by a conditioning stimulation was also enhanced after injection of CCA. This latter effect was suppressed in a reversible manner by injection of bicuculline. These results strongly suggest that the effect of CCA upon electrical activity of Purkinje cells is related to an enhancement of the inhibition exerted on Purkinje cells by GABAergic, cerebellar interneurones. The possible mechanisms of action of CCA are discussed.