Selective secretion of FSH, involving no apparent changes in LH secretion, was studied in cycling rat females in the late proestrus and early estrus stages. The preovulatory wave of LH and FSH secretion, observed in the second half of the day of proestrus was associated with elevation of the levels of nuclear estrogenic and androgenic receptors. High concentrations of both receptor types were seen during a secondary elevation of FSH secretion early in the morning in the estrus stage. Administration of phentolamine (alpha-adrenoreceptor blocker) did not influence blood levels of FSH and LH in the estrus stage early hours. Administration of a dopamine blocker haloperidol inhibited the second wave of FSH secretion, this being paralleled by a reduction of the number of estradiol nuclear receptors. The authors suggest that estrogenic and androgenic receptors of adenohypophysis and dopaminergic systems of the brain contribute to the mechanism of regulation of the second phase of FSH increased secretion observed in the early morning hours of the estrus stage, the dopamine effect on FSH release being mediated via estradiol receptors.