Up to the present time the assay of progesterone from blood, urine and even corpus luteum itself has revealed only minute amounts of this hormone. In 1929 Marrian1isolated from urine a physiologically inactive substance which was identified by Butenandt2in 1930 and called by him pregnanediol. However, it was not until 1936 that Venning and Browne3recognized this substance as an excretion product of progesterone. Using a gravimetric method involving the determination of sodium pregnanediol glucuronidate, in which form pregnanediol is excreted, these investigators have measured the pregnanediol excretion in the normal menstrual cycle and in pregnancy. Their results are expressed in milligrams of pregnanediol excreted in twenty-four hours. In ten patients with normal menstruation they found that sodium pregnanediol glucuronidate appeared in the urine only in the luteal phase of the cycle, i. e., within twenty-four to forty-eight hours after ovulation, and was then excreted