Abstract 1. 1. Intracellular recordings were made from 3 identifiable neurones in the brain of the snail, Helix aspersa; cell E4 was inhibited by acetylcholine, GABA and glutamate; cell E13 was excited by GABA; and cell F30 was excited by glutamate. These 3 compounds were applied iontophoretically. A range of potential antagonists were tested for blocking actions against acetylcholine, GABA and glutamate. 2. 2. Crotethamide, 800 nmoles, was a specific and reversible antagonist at the inhibitory GABA receptor of cell E4. While pentylenetetrazol, 800 nmoles, was a specific and reversible antagonist at the inhibitory glutamate receptor of the same cell. 3. 3. Strychnine, 8 nmoles, was a specific and reversible antagonist for inhibitory acetylcholine receptors on cell E4. Higher doses, 80 nmoles, will in addition block GABA receptors without any effect on glutamate receptors. 4. 4. HA966, 800 nmoles, had no effect on glutamate receptors but did reduce reversibly the responses to GABA and acetylcholine. 5. 5. Picrotoxin, 80–800 nmoles, reduced all 3 inhibitory responses and GABA excitation but had no effect on glutamate excitation. 6. 6. Bicuculline methochloride, 800 nmoles, reduced reversibly both GABA responses and the response to acetylcholine. It had no effect on either glutamate responses. 7. 7. Bemegride, 800 nmoles, reduced reversibly the inhibitory actions of glutamate and acetylcholine but potentiated irreversibly both responses to GABA. 8. 8. Atropine, 80 nmoles, specifically and reversibly reduced the response to acetylcholine but higher doses were non-specific. Tubocurarine, 8 nmoles, altered all the responses and at higher doses had direct excitatory effects on the cells. It is of little use as a specific antagonist on these neurones.