The neuromuscular systems of the closer muscle of the crab, Pachygrapsus craasipes Randall, and of the deep abdominal extensor muscles of the crayfish, Procambarua clarki (Girard), and the rock lobster, Panulirus interruptus (Randall), were investigated with respect to the effectiveness of the inhibitory axons on excitatory processes and on the muscle fibre membranes. In the closer muscle of Pachygrapsus, the muscle fibres showing large electrical responses to the slow axon also showed large inhibitory potentials. Both the slow and, when present, the fast electrical responses in these fibres were greatly attenuated by inhibition. The muscle fibres giving large electrical responses to the fast axon showed no signs of inhibitory innervation. In the abdominal extensor muscle preparations, inhibitory innervation was present and inhibitory stimulation generally caused a sizeable increase in membrane conductance of the muscle fibres, as did gamma-aminobutyric acid. The active membrane responses in these fibres were often selectively eliminated or reduced by inhibition. The excitatory junctional potentials were not markedly attenuated by inhibition, although their rate of decay was increased. No evidence for a presynaptic inhibitory mechanism was found in these muscles. All of the inhibitory effects could be attributed to a post-synaptic increase in membrane conductance. Inhibitory and gamma-aminobutyric acid actions were enhanced by lowering the temperature.