In sympathetic ganglia of the bullfrog a peptide that resembles luteinizing hormone releasing hormone (LHRH) probably functions as the transmitter for the late slow excitatory postsynaptic potential (EPSP), a signal that lasts several minutes: 1) Radioimmunoassays indicated that the lumbar chain of sympathetic ganglia contains 100-800 pg of an LHRH-like peptide, which disappeared 5 days after preganglionic axons were cut, while by this time the LHRH-like immunoreactivity tripled in the spinal nerves proximal to the cut region. 2) Immunohistochemical studies demonstrated the presence of LHRH-like immunoreactivity in numerous nerve terminals around sympathetic neurons. 3) The LHRH-like peptide can be released from ganglia with isotonic KCI or with nerve stimulation, a release that requires Ca2+. 4) LHRH acts directly on sympathetic neurons and produces a depolarizing response lasting for minutes. 5) The LHRH response and the late slow EPSP are associated with similar membrane conductance changes. Further, they alter the excitability of the neuron and interact with the cholinergic EPSPs in a parallel manner. 6) Both responses are blocked by certain analogs of LHRH, which antagonize the effects of LHRH in the rat.