Estrogen and luteinizing hormone preps. inhibited the action of FSH on the ovaries of immature rats when injected for 8 days previous to FSH treatment. In hypo-physectomized rats (immature), estrogen did not produce this inhibition. The inhibiting substance did not produce inhibition by direct action on the ovaries since ovaries from rats treated with estrogen for 8 days responded to FSH when implanted into untreated animals. Ovaries from normal rats implanted into treated animals did not respond. Assay of the blood of rats treated for 8 days with estrogen showed that LH was present in greater conc. than in normal rats or in rats which had received estrogen for only 3 days. Attempts to separate the antagonistic principle from the luteinizing hormone were unsuccessful.