SummaryFemale mice of C3H strain when fed on a diet of 15% Lithospermum ruderale go into an immediate and persistent anestrus. Female mice of Rockland strain fed similarly also go into anestrus which, however, may be followed by varying degrees of developed refractoriness to the drug, as evidenced in some mice by a reappearance of normal estrous cycles. The anestrous condition in both strains is accompanied by atrophy of the ovaries and uteri, and atresia of the follicles. Possible modes of action of the Lithospermum are discussed.