The visceral muscles of the oviducts of Locusta migratoria are sensitive to the pentapeptide proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH). Amounts of proctolin as low as 2 fmol induce a tonic contraction that is dose-dependent up to 200 fmol. By use of this bioassay we have quantified the amount of material showing proctolinlike bioactivity associated with the oviducts. Reversed-phase high performance liquid chromatography of tissue extracts indicates that material with proctolinlike bioactivity and co-eluting with proctolin is present in the oviducts, the oviducal nerve, and the VIIth abdominal (penultimate) ganglion. The proctolin is present in areas of oviduct that receive extensive innervation. There is tenfold less proctolin in areas of oviduct that receive little or no innervation. Proctolinlike immunoreactivity is present in axons of the oviducal nerve as well as in a number of cell bodies in the VIIth abdominal ganglion. Three of these neurons lie in a position similar to that of the previously described oviduct motoneurons. Neuropilar axons and processes, as well as axons in the median nerve, also show proctolinlike immunoreactivity. The results indicate that proctolin, which has previously been identified as a neurotransmitter of insect hindgut visceral muscle, is also associated with visceral muscle of the reproductive system.