Proctolin-like immunoreactivity (PLI) was extensively distributed within the central and stomatogastric nervous systems and gut of Locusta migratoria. PLI was also found in the lateral neurosecretory cells, the nervi corporis caradiaci II, corpus cardiacum (CC), and corpus allatum (CA). In contrast, in the cockroach, Diploptera punctata, PLI was associated with the CC, but was absent within the CA.The control of adipokinetic hormone I (AKH I) and juvenile hormone (JH) release by proctolin was investigated. Proctolin caused dose-dependent increases in AKH I release from the locust CC, with threshold at 10-8 M and maximal response at approximately 10-7 M proctolin. Confirmation of AKH release was obtained by bioassay for adipokinetic activity. Proctolin significantly increased the release of JH from the locust CA. Proctolin did not alter the release of JH from the cockroach CA. The PLI coupled with physiology suggests proctolin as a releasing factor for locust AKH I and JH but not for JH from the CA of cockroach.