When penicillin, an epileptogenic agent, was applied to the neuromuscular junctions of the superficial flexor muscles of crayfish, the excitatory junctional potential (EJP) amplitudes were increased by 50-200%. This effect of the drug was not due to changes in the passive electrical properties of the muscle cell membrane or to an increase in its chemical sensitivity to acetylcholine (ACh), the presumed transmitter at the junction studied. Inactivating the penicillin with the enzyme penicillinase, or substituting acetate for penicillin in the test solutions eliminated the effect on EJPs, showing that the penicillin ion was the active agent. Penicillin ions did decrease the frequency of spontaneous miniature EJPs and increase the amplitude or presynaptic spikes recorded extracellularly, suggesting that augmentation of EJPs may have been due to alterations at the presynaptic nerve terminals.