Several possible inhibitory transmitters were tested iontophoretically on cerebellar Purkinje (P.) cells. GABA had the strongest action, consistently blocking the simple spike discharge of P. cells when applied with small ejecting currents. The same potency was shown by GABOB. Glycine was clearly much weaker than GABA, as were β-alanine, δ-aminovaleric acid and ɛ-aminocaproic acid. Taurine was practically devoid of any activity. Noradrenaline and dopamine had a weak and generally slow depressant action on a variable portion of the P. cells. The effects of serotonin were even more inconsistent. The depression induced by these compounds was resistant to strychnine and picrotoxin, administered either iontophoretically or i.v., in agreement with the properties of the inhibition of P. cells induced by electrical stimulation of parallel fibres. The results are consistent with the possibility that GABA, or some closely related compound, is the transmitter released by the cerebellar inhibitory interneurones acting on P. cells.