Sprague-Dawley rat pups at 3-4 days prenatally were tested in both the absence and presence of milk following administration of various doses of either the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), the 5-HT1B agonist 1-(3-chlorophenyl)piperazine (mCPP), or the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Administration of 8-OHDPAT decreased mouthing, increased probing and increased behavioral activation. Conversely, the 5-HT2 agonist DOI and the 5-HT1B agonist mCPP increased mouthing and decreased probing. mCPP and DOI differed in their effects on behavioral activation, with mCPP decreasing and DOI increasing this composite behavioral score. mCPP increased grooming, whereas DOI elicited a characteristic unusual positioning of the limbs. Thus it appears that 5-HT1A, 5-HT1B and 5-HT2 receptor subtypes are present in the neonate and elicit differential behavioral responses upon stimulation with selective agonists. Ontogenetic variations in the balance among these receptor subtypes during development may be related to the ontogenetic reversal that has been previously reported in the impact of serotonin manipulations on mouthing and suckling behavior during the neonatal to weanling age period.