The regional distribution of the 4 neuropeptides, bombesin, substance P, somatostatin and vasoactive intestinal polypeptide (VIP) were investigated in the developing rat brain. Specific radioimmunoassay and immunocytochemistry were employed. VIP and bombesin were undetectable in the foetal brain whereas substance P and somatostatin were shown to be present in all regions as early as 14 days postcoitus. There was a dramatic postnatal increase in all 4 peptides in most regions. These results are discussed and compared with results of previous investigations of the ontogeny of the classic neurotransmitters.