The progravid phase of the rat reproductive cycle, or pseudopregnancy, was induced by mating with vasectomized males. The morning on which a copulation plug was observed was designated day one. Between 1100 and 1200 hr on each of days one through 13, rats were decapitated and the blood collected in heparinized test tubes. The plasma content of progesterone and of 20α Hydroxypregn–4–en–3–one (20α–OHP) was measured using a competitive protein binding assay, after the separation of competing steroids by Sephadex—LH 20 chromatography. The progesterone levels increased strikingly between day 2 and day 4, were maximal between day 4 and day 7, but had not returned to baseline values by day 13. The 20α–OHP levels decreased steadily from days 1–day 9. Between day 10 and day 11 there was a significant increase in 20α–OHP levels. Between days 1 through 7, the ratio: progesterone/20α–OHP changes in a pattern which is quite similar to the pattern of changing progesterone levels. The ratio: progesterone/ 20α–OHP was observed to increase between day 7 and day 10 and to abruptly decrease between day 10 and day 11. It is suggested that: 1) the adrenal contribution to the total progesterone observed in plasma during the latter days of the rat's progravid cycle is appreciable; 2) the duration of the life span of the progravid corpus luteum of rats cannot be accurately assessed by measuring the progesterone content of peripheral plasma; 3) the abrupt increase in the concentration of 20α–OHP observed between day 10 and day 11 is probably due to a concomitant abrupt increase in the luteal content of the enzyme, 20α–hydroxy—steroid—dehydrogenase, (20α–OH—SDH); and 4) the induction of enzyme activity, (20α–OH—SDH) within any given corpus luteum is an irreversible event. “Physiological luteolysis” is a phrase which is proposed to characterize the event of an irreversible decrease in luteal progesterone secretion. In the peripheral plasma of progravid rats, the day on which this event occurs can be inferred from the abrupt and irreversible decrease in the ratio: progesterone/20α–OHP. (Endocrinology92: 949, 1973)