Previous experiments performed in rats under barbiturate anaesthesia have shown a remarkable increase of simple spike firing rate in cerebellar Purkinje cells following inferior olive lesion or inactivation. The increase is due, at least in part, to the withdrawal of the tonic background activity of corticocerebellar interneurones, which have GABA as a chemical transmitter. Since barbiturates potentiate GABAergic inhibition, it is possible that the effect is due to the presence of this type of anaesthesia. In absence of general anaesthesia, we have compared the simple spike firing rate of the Purkinje cells in rats with intact inferior olive and 3-5 days after inferior olive lesion by 3-acetylpyridine. In the latter condition, the firing rate is significantly higher. In other rats, under urethane anaesthesia, which is not known to interfere with GABAergic transmission, the inferior olive has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Following cooling of the inferior olive on one side, a remarkable increase of simple spike activity, parallel to the disappearance of complex spike activity, has been observed in the Purkinje cells of the contralateral side. These results show that the presence of the simple spike firing increase, which follows the removal of the climbing fibre activity, does not depend on an anaesthetic which potentiates GABAergic transmission, although its amplitude is affected by the same anaesthetic. They suggest, therefore, that the tonic inhibition exerted by the olivocerebellar pathway on the Purkinje cells operates also in physiological conditions.(ABSTRACT TRUNCATED AT 250 WORDS)