In summary, progesterone is probably the only naturallly occurring progestational agent of any significance. Small amounts may be synthesized by the cells of the follicle in the provulatory swelling phase; however, the major production is by the corpus luteum cells of the ovary during the luteal phase of the cycle. It is constantly produced in small amounts by the adrenal gland and by the testis in the male. In the adrenal and the testis, it probably serves as the precursor for corticoids and androgens. It is transported in the blood by a specific binding protein and metabolized and conjugated in the liver into sodium pregnanediol glucuronide which also circulates in the blood. Approximately 20% is excreted in the urine as sodium pregnanediol glucronide; pregnanolone represents a minor metabolic product. The pregnanediol which is excreted in the bile is enzymically hydrolyzed by the gut so that the pregnanediol recovered in the feces is in the free form.