The effect of chronic administration of bombesin (BBN) on the development of the pancreas and the role of endogenous BBN in the postnatal development of exocrine pancreatic function were investigated in suckling New Zealand White rabbits. BBN administered intraperitoneally (i.p) at various doses (1.25, 12.5 and 30 micrograms/kg body weight) to suckling rabbits for 13 days starting on day 4 of life induced pancreatic growth as characterized by dose-dependent increases in pancreatic wet weight, total protein content and total DNA content compared to littermate controls. The maximum effect was observed using bombesin at 30 micrograms/kg. This increase represented hyperplasia since there was no BBN-induced change in the protein:DNA ratio. Pancreatic amylase activity was significantly increased by all doses of BBN, with a maximal effect at 1.25 micrograms/kg. The specific BBN receptor antagonist [Leu13-psi(CH2NH)Leu14]-BBN, given i.p for 9 days at 30 micrograms/kg body weight starting on day 20 of life, significantly reduced the development of pancreatic amylase and lipase activities compared to controls (48 and 36% reductions, respectively). Our findings confirm the trophic effect of BBN on the neonatal pancreas and provide evidence of a physiological role for endogenous BBN-peptides in the development of pancreatic exocrine function.