The effects of prenatal stress on the immunocytochemical reaction for serotonin (5-HT) in the dorsal raphe nucleus (DRN) were studied in seven-day-old male rat pups subjected to pain in the formalin test (inflammatory pain; controls received injections of physiological saline. Prenatal stress enhanced the animals’ pain behavior in the formalin test. Prenatal stress decreased the numbers of 5-HT-immunoreactive cells as compared to those in prenatally unstressed animals, both in rat pups with inflammatory pain and in controls. The formalin treatment did not alter the number of 5-HT-immunoreactive cells in the DRN in rat pups in either prenatally stressed or unstressed animals. Thus, we demonstrate that 5-HT-ergic neurons in the brainstem DRN in seven-day-old rat pups are the targets of prenatal stress. We suggest that stress due to separation from the mother during the experiment masked differences in the immunocytochemical parameters in rat pups with inflammatory pain and in controls.