Microiontophoretic application of harmaline evoked rhythmic multiunit activity in the inferior olive of decerebrate cats. Harmaline caused strong excitation of individual olivary neurones but did not seem to cause them to discharge in high frequency bursts. These effects suggest that the tremorgenic action of harmaline may be due to an exaggeration of the normal tendency of olivary neurones to fire rhythmically in multiunit bursts.