Rowlands & Williams [1943] recently reported that the consecutive injection of mare-serum gonadotrophin and chorionic gonadotrophin would cause ovulation in hypophysectomized rats. Serum gonadotrophin was given to stimulate the growth of the atrophic follicles and, subsequently, chorionic and other gonadotrophins containing an excess of the luteinizing hormone were used as a means of causing their rupture. Ova were found in the Fallopian tubes 1–2 hr. after their discharge from the ovaries. Similar experiments have been carried out in the intact immature rat to study, in greater quantitative detail, the optimal conditions necessary for ovulation. A comparative study of the hormonal control of ovulation in different species seemed desirable, particularly on account of the disappointing results that have been frequently reported on the use of mare-serum gonadotrophin in causing ovulation in women. MATERIAL AND METHODS Gonadotrophic substances Two preparations of mare-serum gonadotrophin were used: (1) the International Standard containing 4 International