The disappearance of progesterone* and HCG from the plasma was measured following therapeutic abortions in 17 patients from the seventh to the sixteenth week of pregnancy. Between the seventh and the twelfth week of gestation the evacuation of the uterine contents was performed by vacuum aspiration, and from the thirteenth week onwards abdominal hysterotomy was performed. Plasma progesterone was assayed using a competitive protein binding technique, while plasma HCG was determined by a radioimmunosorbent assay. After the evacuation of the uterine contents in the early pregnancy cases (weeks 7–8) the plasma levels of progesterone remained elevated much longer than in the later stages of pregnancy (weeks 12–16), thus suggesting that the corpus luteum graviditatis was the main source of progesterone in the first group, while in the last group the placenta was the main source of progesterone production. When comparing the earlier and later cases of pregnancy, the disappearance rate of plasma HCG remained essentially unaltered. From the results obtained, it seems likely that the transition from the ovarian to the placental production of progesterone occurs during the ninth to eleventh weeks of pregnancy.