Progesterone concentrations in 2 full-term normal placentas, molar tissue from 3 cases of hydatidiform mole, and ovarian tissue from a theca lutein cyst of hydatidiform mole were measured by competitive protein binding. Sephadex LH-20 column chromatographic separation of extracts from these tissues showed that, in placental and molar tissue, 85 to 95% of the extracts were progesterone and very small amounts of 20α-dihydroprogeslcrone and 17α-hydroxyprogesterone. The concentrations of progesterone in the placentas were 368.8 and 317.2 ng/g tissue, respectively, while in molar tissue the concentrations were 2474.5, 1974.6, and 4146.0 ng/g tissue, respectively. Progesterone concentration in ovarian tissue was between 1121.2 and 1440.9 ng/g tissue. It is suggested that the high concentration of progesterone in molar tissue reflects functional capacity of the abnormal trophoblast in progesterone synthesis and accumulation of progesterone due to absence of a fetus. The lower concentration of progesterone in the ovary in molar pregnancy would appear to suggest that the ovary is a secondary source of progesterone in molar pregnancy.