The metabolic clearance rate (MCRrB), plasma concentration, and production rate (PI) and conversion to progesterone of 20α-hydroxypregn-4-en-3-one (20α-OHP) have been determined in women during the menstrual cycle. The MCRrB was 3,110 ± 217 (SE) 1/day and was independent of the day of the cycle. The PI was 0.92–2.53 mg/day for days 6–18 of the cycle and 0.78–15.6 mg/day for days 20–24. The percent conversion of 20α-OHP to progesterone was 4.88 ± 1.10 (SE)% and the calculated transfer constant of 20α-OHP to progesterone was 0.05. The contribution of progesterone to the blood production rate of 20α-OHP for days 20–24 was calculated to be between 24 and 73 %, and it was concluded that plasma concentrations of 20α-OHP are not a valid indicator of corpus luteum regression in the human.