The M-current of dissociated bullfrog sympathetic neurons, measured by the whole-cell patch clamp technique, is powerfully inhibited by chicken II LH-RH (luteinizing hormone-releasing hormone), with 50% inhibition near 1 nM. Chicken II LH-RH is approximately 100 times more potent than salmon LH-RH, and at least 1000 times more potent than other known naturally occurring LH-RH analogs (chicken I LH-RH, mammalian LH-RH, and lamprey LH-RH). This high potency makes chicken II LH-RH a candidate for the endogenous transmitter mediating the late, slow excitatory postsynaptic potential (EPSP) in bullfrog sympathetic ganglia.