Proctolin-like immunoreactivity (PLI) was widely distributed in the locust, Locusta migratoria, within the central, peripheral and stomatogastric nervous systems, as well as the digestive system and retrocerebral complex. Proctolin-like immunoreactivity was observed in cells and processes of the brain and all ganglia of the ventral nerve cord. Of interest, PLI was found in the lateral neurosecretory cells, which send axons within the paired nervi corporis cardiaci II (NCC II) to the corpus cardiacum (CC). The CC contained extensive processes displaying PLI, which continued on within the paired nervi corporis allata (NCA) to the paired corpora allata (CA) where the axons entered and branched therein. The frontal and hypocerebral ganglia of the stomatogastric nervous system contained PLI within processes, resulting in a brightly staining neuropile. Each region of the gut contained PLI in axons and processes of varying patterns and densities. The paired ingluvial ganglia contained PLI, including an extensively stained neuropile and immunoreactive axons projecting through the nerves to the foregut. The hindgut contained PLI within longitudinal tracts, with lateral projections originating from the 8th abdominal ganglion via the proctodeal nerve. The midgut contained PLI in a regular latticework pattern with many varicosities and blebs. No difference in PLI in cells and processes of the central nervous system (CNS) was found between males and females.