LHRH perikarya and processes were compared in 6, 9, and 11 day old normal and estrogenized male and female rats. Estradiol benzoate was administered SC in 1 or 1000 microgram amounts when the animals were 2 days old. Control animals received either an equivalent amount of the oil vehicle or no injection. After perfusion with Bouin's solution, brains were embedded in paraffin and sectioned 6 mu coronally through the entire brain. The peroxidase-antiperoxidase immunocytochemical technique was used to assess the development of LHRH cells and fibers. The primary antiserum to LHRH was Arimura's antiserum #743. LHRH immunopositive perikarya were present in the septal-preoptic but not in the arcuate-median eminence regions. No immunopositive reaction product was present following treatment with antiserum #743 absorbed with LHRH. At each of the three ages, the amount of detectable LHRH cell bodies was considerably greater in normal males than in females. The high dose of estrogen reduced the quantity of LHRH perikarya localized in the male and increased it in the female. LHRH processes formed a "rostral" pathway in proximity with the organum vasculosm of the lamina terminalis (OVLT) in the vicinity of the rostral preoptic area and a "caudal" one extending through the arcuate-median eminence region. Many more LHRH immunopositive processes were visualized in normal males than in female siblings on days 9 and 11. The high dosage of estrogen also reduced the amount of LHRH immunoreactive material in 9 and 11 day old males but not in equivalent age females. The data are consistent with the findings that the hypothalamic-pituitary-gonadal inhibitory system functions earlier in the male than in the female and that the LHRH system can be modified by estrogen circulating during the period of sexual differentiation.