1. Potential changes between the pial and cut surfaces of slices of guinea-pig olfactory cortex in vitro produced by gamma-aminobutyric acid (GABA) were recorded with extracellular electrodes. 2. GABA, superfused over the pial surface (0.1 to 10 mM), produced a pial-negative potential deflection, accompanied by inhibition of the postsynaptic response to lateral olfactory tract (LOT) stimulation. 3. This effect was replicated by the following compounds (potency relative to GABA = 1, in brackets): 3-aminopropanesulphonic acid (5.3), epsilon-aminovaleric acid (0.07), beta-alanine (0.07), beta-amino-nibutyric acid 0.05), epsilon-aminocaproic acid, alpha-amino-isobutyric acid, L-leucine (less than 0.02). 4. L-Glutamate (1 to 10 mM) produced a very large surface negative shift, with relatively less synaptic inhibition. Glycine (1 to 10 mM) produced less surface negatively, accompanied by synaptic inhibition. 5. Responses to GABA were antagonized more effectively than those to glycine by bicuculline (3 to 30 micrometer) and picrotoxin (1 to 30 micrometer). Strychnine (1 to 10 micrometer) incompletely inhibited responses to glycine. 6. It is concluded that, while the locus within the slice of these effects is uncertain, the preparation may be useful for testing the interaction of drugs with cerebral GABA receptors.