As early as 1926, Teel (1) found that gestation was prolonged in rats injected with a crude alkaline extract of beef hypophyses. He stated that the fetuses lived up to or beyond term and then died within the uterus due to a failure of the birth mechanism. Hain (2), D'Amour and Dumont (3), Evans and Simpson (4) have obtained the same results with relatively crude unfractionated hypophyseal extracts. Hain (2) reported prolonged gestation with phyone, but Watts (5) showed that GH free of gonadotropic substances would not inhibit parturition. Hoopes and King (6) prevented parturition with prolan B. It seems, therefore, that this action is associated with the gonadotropic principle, but it is still not known whether parturition is inhibited by the gonadotropic principle directly or whether it acts indirectly through the ovaries. It was hoped that the following experiments would contribute some further data as to the manner in which the birth mechanism is disturbed by gonadotropic substances.