gamma-aminobutyric acid (GABA) is the inhibitory transmitter compound at the lobster neuromuscular junction. This paper presents a comparison of the enzymes of GABA metabolism in single identified inhibitory and excitatory axons from lobster walking legs. Inhibitory axons contain more than 100 times as much glutamic decarboxylase activity as do excitatory axons. GABA-glutamic transaminase is found in both excitatory and inhibitory axons, but about 50% more enzyme is present in inhibitory axons. The kinetic and electrophoretic behavior of the transaminase activity in excitatory and inhibitory axons is similar. Succinic semialdehyde dehydrogenase is found in both axon types, as is an unknown enzyme which converts a contaminant in radioactive glutamic acid to GABA. In lobster inhibitory neurons, therefore, the ability to accumulate GABA ultimately rests on the ability of the neuron to accumulate the enzyme glutamic decarboxylase.