A potent LH-RH antagonist (Ac-D-Trp1,2, D-Cpa2, D-Lys6, D-Ala10 LH-RH (Antag) was used to study the differential regulation of FSH and LH secretion by endogenous LH-RH in ovariectomized (OVX) and regularly cycling rats. The endogenous LH-RH was suppressed by single injections of Antag in OVX animals and by long-term treatment with Antag in normal and OVX rats. Serum and pituitary LH and FSH, as well as serum estradiol (E2) and progesterone (P) was determined by RIA during and/or after the treatment. The direct effect of the antagonistic analog on the ovarian P release was tested in vitro using the isolated luteal cell system. Single injections of the Antag in OVX animals caused prompt and marked suppression of the serum LH (-80%), while no decrease of the serum FSH. Long-term treatment with the same analog decreased the serum LH by 50% but did not modify the serum FSH. In normal rats, serum LH dropped to undetectable levels, while serum FSH did not change significantly. Long-term treatment with the antagonist also resulted in divergent alterations in the pituitary gonadotropin concentrations. In OVX animals, the pituitary LH content moderately elevated (+21%), however, the FSH did not change. In normal rats, ovarian cycles were interrupted, and no ovulation appeared during the treatment. The pituitary LH concentration increased by 46%, while the FSH decreased by 43%. Marked depression was found in the serum P (-60%) but no significant change in the serum E2 levels. Incubation of isolated luteal cells with the Antag did not influence the HCG-induced P secretion in vitro, demonstrating that the in vivo inhibitory effect of the antagonistic LH-RH analog on the P secretion asserts not directly on the ovarian LH-RH receptors, but through inhibition of the endogenous LH-RH. Our studies give evidence that the long-term treatment with LH-RH antagonist suppress the LH and P but not the FSH and E2 secretion, and provide new data suggesting the presence of a FSH-releasing factor in the CNS.