Synthetic LH-RH (0.75 and 2.5 mug/animal) was administered to mature intact and castrated female rabbits. The animals were injected for 3 consecutive days with either 40 mug estradiol benzoate (EB) dissolved in 0.5 ml propylene glycol 60%, water 40% or with the solvent alone. LH-RH was injected i.v. on the 4th day. In intact rabbits EB priming increased the pituitary LH response while LH basal levels were lowered. In the unprimed castrated rabbits, basal FSH and LH levels and the cumulative responses to LH-RH were higher than in intact control animals; EB priming lowered basal LH levels and cumulative responses to the values of intact control rabbits while both basal FSH levels and the cumulative responses were less influenced by the treatment. It is concluded that estrogen acts at both hypothalamic and pituitary levels to modify the LH responses to endogenous LH-RH, and that the complete inhibition of FSH hypersecretion in the ovariectomized rabbit probably needs another ovarian factor.