1. The effect of picrotoxin on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The potential changes were recorded intracellularly and extracellularly with micro-electrodes. The membrane conductance of the muscle fibre was also measured.2. Picrotoxin depressed the amplitudes of the inhibitory junctional potentials and the potential changes produced by iontophoretically applied gamma-aminobutyric acid (GABA), but had no appreciable effect on the excitatory junctional potentials and the potential changes produced by L-glutamate.3. The presynaptic action of GABA and the neural transmitter was depressed by picrotoxin. The presynaptic action of beta-guanidinopropionic acid was also depressed by picrotoxin.4. The increase in the membrane conductance produced by the addition of GABA in the bath fluid was depressed by picrotoxin. The dose-response relation showed that picrotoxin depressed the conductance increase produced by GABA in a non-competitive manner. The action of picrotoxin on the conductance increase produced by GABA was more effective in low Cl- solution.5. The analysis of the dose-response curves showed that the action of picrotoxin was well expressed by the Michaelis-Menten equation, but the slope of the dose-response curve of GABA was steeper than this relation. It is proposed that the conductance of the junctional membrane was increased by the combination of two molecules of GABA with a receptor, and the attachment of one molecule of picrotoxin to a specific site depressed the conductance increase.