SUMMARY The action of pineal extracts on the genital system of several species of laboratory animals has been investigated. Determination of the phosphorus turnover in the genital organs of treated animals proved to be a significant guide to the action of pineal preparations. Doses of pineal extracts lower than those required for weight changes sufficed to produce changes in phosphorus metabolism. Apparently contradictory results encountered originally were explained when findings were evaluated on the basis of the age of the test animal and the type of extract used. Thus, in immature and/or hypophysectomized animals, crude aqueous extracts stimulate the genital system, whereas in mature intact animals the same extract often has an inhibitory effect on the same organs. The inhibitory effect could be measured also in immature mice treated with gonadotrophic hormone and this proved useful in short-term experiments. Crude glandular suspensions apparently contain both inhibitory and stimulatory principles. Partial separation of these was achieved through the use of trichloracetic acid, the supernatant fluid having predominantly inhibitory action while the stimulatory substance was found mainly in the precipitate.