A rabbit antiserum to synthetic LHRH was shown to be specific to rat hypothalamic LHRH, as revealed by binding studies (RIA). The present study was undertaken to determine the effects of this antiserum (LHRH-AS) on the progesterone-induced LH surge in ovariectomized rats primed with estrogen on the proestrous LH surge in cycling females. An ip injection of 0.2 ml LHRH-AS at 1200 h on proestrus completely inhibited the preovulatory rise in serum LH from 1700-1800 h. In long term ovariectomized rats, a sc injection of estradiol benzoate reduced serum LH levels by 50% at 1200 h on the third day. Subsequent administration of progesterone elicited a LH surge, with peak LH value 4-6 h after progesterone. However, concurrent administration of LHRH-AS effectively prevented the increase in LH release by progesterone, and an iv injection of LHRH-AS was shown to be more effective than ip administration in inhibiting the progesterone stimulation of LH release. Although estrogen and progesterone may induce a change in pituitary responsiveness to hypothalamic LHRH in ovariectomized rats, the present study provides evidence that tonic and/or increased LHRH release is required for the acute increase in LH release caused by progesterone.