SUMMARY1. A slow synaptic potential recorded in neurones of sympathetic ganglia of bullfrog, the late slow e.p.s.p., is probably mediated by a peptide resembling luteinizing hormone-releasing hormone (LHRH), because (1) a LHRH-like peptide is contained in preganglionic nerve terminals and is released upon nerve stimulation, (2) application of LHRH to ganglion cells mimics the effects of the natural transmitter for the late slow e.p.s.p., and (3) the pharmacological properties of LHRH and the natural transmitter for the late slow e.p.s.p. are similar.2. Neurones in frog sympathetic ganglia are also depolarized by substance P. The