The plasma levels of FSH and LH in mitomycin C treated animals have been estimated by radio-immunoassay technique. The results indicate that mitomycin C in non-toxic doses stimulates the pituitaries to increased production of gonadotrophins probably by directly inhibiting ovarian steroidogenic activity. This direct effect of the antibiotic on the ovaries is further confirmed by experiments on immature rats and the in vitro studies on ovarian slices.