Sexual maturation and differentiation of the rat brain are believed to result from the interaction of gonadal steroids with specific neural receptors during late fetal and early postnatal life. A variety of evidence indicates that the first appearance of estrogen receptors in the hypothalamus-preoptic area (HPOA) during the perinatal period is a crucial evident underlying these processes. However, it is unknown to what extent the ontogeny of estrogen receptors is itself influenced by gonadal steroids present in the fetal environment. In order to address this question, estrogen receptors were assayed in HPOA 8 weeks after transplantation of the tissue from embryonic day 15 to 18 fetuses to either the choroidal pia overlying the superior colliculus or to the anterior chamber of the eye of adult female hosts. Host animals were either intact or ovariectomized and adrenalectomized, with or without estrogen replacement. The saturable binding of estradiol to cytosol of HPOA transplants exhibited the steroid specificity and high affinity characteristic of authentic estrogen receptors. No differences in the level of cytosol estrogen receptors in transplanted HPOA grown in the presence or absence of gonadal steroids were found. Receptor concentrations were also similar in HPOA taken from male or female fetuses. Autoradiography with [3H]estradiol revealed clusters of estrophilic cells in the transplants similar to those of the adult host hypothalamus, again regardless of whether the transplant developed in the presence of gonadal steroids. Estrogen receptors from both groups were also found to be biochemically functional as indicated by the ability of acute estrogen treatment to induce progestin receptors in the transplants.(ABSTRACT TRUNCATED AT 250 WORDS)