Stimulation of the parallel fibres, or the mossy fibres, in the cerebellar cortex depresses the potential generated in the granular layer by anti-dromic invasion of the Purkinje cells (N1), and that generated by the axon discharge of granule cells (P2). The reduction of these potentials indicate inhibitions mediated respectively by basket and Golgi cells. The depressions of both N1 and P2 potentials are unaffected by strychnine at doses of up to 1 mg/kg. Picrotoxin and bicuculline reduce or suppress both inhibitions at doses of 2 to 5 mg/kg and 0.2 to 0.4 mg/kg respectively. The action of the picrotoxin is long lasting or even possibly irreversible, whereas that of bicuculline lasts only a few minutes. The ratio by weight between the dose of picrotoxin and that of bicuculline necessary to reduce the N1 and P2 depression exceeds 10. These results indicate that gamma-aminobutyric acid may be the chemical inhibitory transmitter at basket cell — Purkinje cell and Golgi cell — granule cell synapses.