Litters from stressed and control females were cross-fostered at birth to determine whether the effects of maternal stress on the offspring originated prenatally or during the neonatal period. Offspring of stressed females reared by control mothers from birth showed a reduced behavioral response to injections of the 5-hydroxytryptamine (5-HT) agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), increased 5-HT2 receptor binding in cerebral cortex and increased open field activity when tested at 60 days of age. In contrast, control litters reared by previously stressed females showed an increased behavioral response to 5-MeODMT, increased 5-HT2 receptor binding and only minor changes in open-field activity. These results provide further evidence that adult rat behavior can be significantly altered by exposure to the effects of maternal stress in utero. However, the effect of maternal stress on central 5-HT receptors is also strongly influenced by the postnatal rearing conditions.