Proctolin was the first insect neuropeptide to be sequenced and has been the subject of many physiological and pharmacological studies in insects and crustaceans. We have identified a Drosophila gene (CG7105, Proct) encoding a precursor protein containing the neuropeptide proctolin (RYLPT). In situ hybridization with a riboprobe to the Proct gene revealed a distribution of transcript in neurons of the larval central nervous system (CNS) matching that seen with antiserum to proctolin. An antiserum raised to a sequence in the precursor downstream of proctolin labeled the same neurons as those seen with the antiproctolin antisera. The predicted protein encoded by Proct has a single copy of the RYLPT sequence that directly follows the predicted signal peptidase cleavage point and precedes a consensus recognition site for a furinlike processing endoprotease. Ectopic expression of Proct in the CNS and midgut via the GAL4-UAS system, using an Actin5C-GAL4 driver, confirmed that the predicted preproproctolin can be processed to generate immunoreactive proctolin peptide. Pupae over-expressing Proct displayed a 14% increase in heart rate, providing evidence in support of a cardioacceleratory endocrine function for proctolin in Drosophila. The distribution of proctolin suggests roles as a neuromodulator in motoneurons and interneurons, and as a neurohormone that could be released from brain neurosecretory cells with terminations in the ring gland. (C) 2003 Wiley Periodicals, Inc. J Neurobiol 58: 379-391, 2004.