The possible involvement of serotoninergic mechanisms in the induction of harmaline generated tremor in the inferior olive has been investigated electrophysiologically in the cat and rat. Mass recordings of Purkinje cell activity in the cat showed that harmaline induces strong, synchronous and rhythmic activity in those parts of the climbing fibre system originating in the caudal part of the medial accessory olive and the caudolateral parts of the dorsal accessory nucleus. These are the areas of the cat olive shown to receive a dense serotoninergic innervation. In the rat, the selective removal of the serotoninergic innervation--produced by an intraventricular injection of 5,6-dihydroxytryptamine, or 5,7-dihydroxytryptamine in combination with desipramine--caused a significant attenuation of both the tremor and the climbing fibre activity induced by an intravenous harmaline injection. In the 5,6-dihydroxytryptamine-treated animals the reappearance of the harmaline tremor seemed to parallel the regrowth of new serotoninergic axon sprouts in the inferior olive. On the basis of the present results it is proposed that the serotoninergic afferents to the accessory olivary nuclei are of critical importance for the tremor induction of harmaline in the inferior olive. It is suggested that harmaline, rather than acting directly on the olivary neurones, exerts its effect through an interference with a serotoninergic (possibly inhibitory) innervation of these cells.