The recovery of urinary pregnanediol after the injection of progesterone was studied in seventeen women and four men.In non-pregnant subjects less than 20% of the injected progesterone was recovered as urinary pregnanediol. The recovery did not increase significantly when progesterone was given to pregnant women, to women in the luteal phase of the cycle or after prolonged administration to postmenopausal women. The 'progesterone priming' effect described by Sommerville & Marrian [1950] was not observed. ' The catabolic fate of the hormones may be completely unrelated to their unique biological functions.' This remark of Lieberman & Teich [1953] sums up the experience of many workers in the field of the metabolism of steroid hormones. It is particularly applicable to the study of progesterone metabolism. Progesterone is known, in humans, to be in part reduced to pregnanediol, some of which is excreted in the urine. It is also known that progesterone exerts a profound effect on the functions of the uterus, such as endometrial secretion, uterine contraction and the nidation of the ovum. Since these two observations were made, many attempts have been under¬ taken to establish a connexion between them. At first it was thought that the endometrium was essential to the conversion of progesterone to pregnanediol [Venning & Browne, 1937; Hamblen, Powell & Cuyler, 1939]. This was disproved when it was shown that hysterectomized women and normal men could excrete pregnanediol after the administration of progesterone [Buxton & Westphal, 1939;Venning & Browne, 1940]. Venning & Browne [1940] then produced evidence to show that in the presence of secretory endometrium or placenta the percentage of administered progesterone excreted as urinary pregnanediol was increased. They used small doses of progesterone (5-30 mg/day), and a relatively inaccurate and insensitive pregnanediol assay method [Venning, 1937]. It is doubtful whether much reliance, from the quantitative point of view, can be placed on results obtained by the Venning method at the low levels of urinary pregnanediol studied in this investigation. The same cannot be said of the subsequent work of Sommerville & Marrian [1950]. They used larger amounts of progesterone (60-120 mg/day), and a more sensitive assay method. They claimed that not only did the pregnant woman excrete a larger proportion of a dose of progesterone as urinary pregnanediol but that the post¬ menopausal woman also showed this phenomenon after she had been exposed to the action of progesterone for 6-8 days, even when the latter was administered orally.