The development of the hypothalamic serotoninergic system has been studied in rats from the 14th fetal till the 9th postnatal day. Serotoninergic elements were detected with immunocytochemistry using antiserum to serotonin (5-HT) and radioautography following the injections of [3H]5-HT into the cerebral ventricles. Immunocytochemistry failed to recognize hypothalamic 5-HT neurons either in fetuses or in neonatal rats. Conversely, radioautography demonstrated sparse radioactively labeled cells, undifferentiated in appearance, at the 16th and 18th fetal days. Moreover, at the 18th fetal day an accumulation of radioactively labeled typical neurons appeared in the suprachiasmatic region. By the 9th postnatal day, practically all radioactively labeled cells looking like highly differentiated neurons were concentrated in the dorsomedial nucleus. As to nerve fibers, a small number of 5-HT-immunoreactive axons arising from the raphe nucleus were first detected in the hypothalamus at the 16th fetal day. Two days later, both 5-HT-immunoreactive and radioactively labeled fibers became widely distributed through the hypothalamus with especially high concentrations in and around the optic chiasma. By the 9th postnatal day, the frequency of 5-HT fibers increased considerably both in the anterior and middle hypothalamus. Some 5-HT fibers were apposed to unlabeled neurons and others abutted on the capillaries either in the organum vasculosum of the lamina terminalis or in the median eminence. The present morphologic study suggests that during the perinatal period the hypothalamic 5-HT system becomes developed enough to be involved in the regulation of some neuroendocrine functions.