Maternal low-level stress was found to produce persistent changes in serotonin (5-HT) receptor binding in several brain regions of the offspring. [3H]5-HT binding was increased in cerebral cortex, decreased in hippocampus and unchanged in pons medulla while [3H]spiperone binding was increased in all three regions at 60 days of age. The binding changes appeared to be due to altered numbers of binding sites with no change in dissociation constants. Regional differences were also found when the ability of nerve terminals to synthesize [14C]5-HT from L-[14C]tryptophan was studied. Prenatal stress reduced the rate of [14C]5-HT synthesis in hippocampus but not in cortex or pons medulla. When younger offspring were studied, binding of [3H]5-HT to cerebrocortical membranes was found to be reduced at 16 days of age and increased at 40 days while [3H]spiperone showed only an increased binding at 40 days. In contrast, prenatal stress resulted in increased nighttime locomotor activity whether measured at 23, 40 or 60 days of age. The present study provides additional evidence that prenatal stress affects the development of serotonergic neurons and it is possible that such changes may underlie the reported behavioural deficits in offspring of stressed female rats.