Using immunohistochemical techniques, we have studied the ontogenetic development of growth hormone-releasing factor (GRF) immunoreactive structures in the rat hypothalamus. Frozen sections of rat hypothalami were stained by the avidin-biotin complex (ABC) method using a specific antiserum against rat GRF. Immunoreactive GRF nerve terminals but not perikarya were first detected in rat fetuses on the 20th day of gestation in the external layer of the median eminence (ME). An increased number of immunoreactive nerve terminals in the ME were observed at 1 and 2 days of age. In addition, perikarya containing immunoreactive GRF-like material were observed in the arcuate nucleus (ARC). Rats at 5 days of age showed a further increase in GRF immunoreactive terminals, which now were also present in the internal layer of the ME. In 10- and 20-day-old rats immunoreactive nerve terminals were only moderately increased in the ME. GRF immunoreactive perikarya were observed in the ARC and also in proximity to the ventromedial nucleus. Moreover, GRF containing fibers were seen projecting from the ARC to the ME. Colchicine treatment of postnatal rats reduced immunostaining of the nerve animals in the ME, but did not affect that of the perikarya. These results are consistent with the view that the neural control of growth hormone secretion develops in the rat during late gestation and continues to mature during the early postnatal period.