The occurrence of substance P-like immunoreactivity was studied in the locust brain at light and electron microscopic level using monoclonal IgG fraction to substance P. Small immunoreactive perikarya have been found beside the medial neurosecretory cells in horizontal brain sections. Widespread immunoreactivity was also observed in the protocerebral neuropil notably in the central body and bordering on the corpora pedunculata. The reaction endproduct appeared as fine, more or less round particles in the central body, and as coarse varicosities and wavy fibres bordering the peduncles. The roundish particles probably represent nerve terminals, while the wavy fibers correspond to neural processes. In the vicinity of the beta lobe immunoreactivity was not observed. Electron microscopically, a number of immunoreactive terminals were found in the protocerebral neuropil. The reaction endproduct was accumulated mostly in large dense core granules/average diameter 80 nm/however reaction endproduct was also observed on the external surface membranes of clear vesicles and mitochondria. Our results suggest the widespread occurrence of a substance-P immunoreactive neuropeptide in the cerebral ganglia of the migratory locust.