Our objective was to find large and identifiable peptide-containing neurons and their postsynaptic targets. For this, a whole mount immunohistochemical method was used to locate cell bodies of neurons in the cockroach central nervous system immunoreactive to antibodies raised against the pentapeptide proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH).The morphology and projections of the immunoreactive neurons were investigated by combining intracellular dye injection and electrophysiological and immunochemical procedures. The presence of proctolin in specific immunoreactive neurons was check by high pressure liquid chromatography combined with bioassay applied to extracts made from individually identified and isolated cell bodies.Using these approaches, we have identified a large proctolin-containing motoneuron which innervates muscles in a proximal segment of the cockroach leg. This study establishes a well characterized cellular preparation in which the physiological actions and functions of a peptidecontaining neuron can be studied in vivo.Proctolin was described initially as a myotropic peptide and proposed as a candidate neurotransmitter in the hindgut or proctodeum of the American cockroach Periplaneta americana (Brown, 1967(Brown, ,1975Brown and Starratt, 1975). Later, it was extracted, purified, and sequenced from 125 kg of whole cockroach (Starratt and Brown, 1975) and shown to have the following structure: H-Arg-Tyr-Leu-Pro-Thr-OH.The presence of proctolin in the proctodeal nerves and the similarity of contractions of the hindgut produced either by nerve stimulation or application of proctolin (Brown, 1975)