In rats under sodium pentobarbitone anaesthesia the inferior olive region has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Unitary activity has been recorded from the fastigial, interpositus and Deiters nuclei. Identification of units was based on the presence of a dye spot, left by the recording micropipette. In the Deiters nucleus, an additional criterion of identification was the antidromic activation from spinal cord stimulation. Following cooling of the inferior olive of one side, we have observed suppression of the activity of all the fourteen Deiters neurones and of seventeen out of twenty neurones recorded from the intracerebellar nuclei. In two out of seven Deiters neurones tested the antidromic invasion elicited by spinal cord stimulation was suppressed. In rats, whose inferior olive was previously destroyed, cooling of the inferior olive region was not followed by the powerful depression of spike activity seen in the vestibular and cerebellar nuclei cells in the intact rats. These results indicate that the olivocerebellar system is very important in regulating the level of excitability of the subcerebellar structures and therefore in controlling both postural mechanisms and the processing of information relating to sensorimotor integration.