Intra-axonal recording was made from the excitatory axon of the lobster walking leg near the nerve terminal and the effects of L-glutamate were studied. Topical application of glutamate to the synapse produced hyperpolarization in the presynaptic membrane with increase in conductance. The glutamate-induced hyperpolarization was reversed to a depolarization at about -100 mV. The reversal potential was not significantly changed by altering the external Cl- but was shifted to more positive values by increasing the external K+. A spider toxin (JSTX), which specifically blocks the postsynaptic glutamate receptor, failed to block the presynaptic glutamate potential. The results suggest that the presynaptic membrane of the lobster neuromuscular synapse has a different type of glutamate receptor from those in the postsynaptic membrane.