Subcutaneous (sc) administration of the tetradecapeptide bombesin (BN) (1-10 mg/kg) elicited grooming in rat pups of 1-10 days of age and the magnitude of this response decreased as a function of age. The form of grooming induced was qualitatively different from that seen following central injection of BN to adult rats. Subchronic neonatal exposure to BN (5 or 10 mg/kg; sc, twice daily for the first 8 postnatal days) had no effect on subsequent adult behavior displayed under mildly stressful or novel conditions, in the open field or in an elevated plus maze. However, both saline and the high dose of BN (10 mg/kg) pretreatments increased adult sensitivity to central BN (0.1 μg; icv) as compared to noninjected but neonatally handled controls or those rats neonatally pretreated with the lower dose of BN (5 mg/kg). This was best demonstrated by increases in scratching activity at the 0.1-μg dose of icv BN. Neonatal pretreatments had no effect on later adult sensitivity to BN injected intraperitoneally (ip). These data indicate that BN receptors in the rat central nervous system are pharmacologically functional from an early stage in ontogeny. Systems utilizing BN-like peptides are, to a degree, plastic early in ontogeny and altered adult sensitivity to BN icv can be achieved via subchronic exposure to BN during infancy. Endogenous BN-based mechanisms did not appear to play a role in the development and/or expression of behavior(s) elicited under mildly stressful or novel conditions.