It is assumed that one function of hCG is to preserve the developing corpus luteum and maintain pregnancy by producing progesterone and thus preventing menstrual shedding. In 8 of 17 cases of ectopic pregnancy, progesterone values were in the range of the proliferative phase of a normal cycle (0.1-1ng/ml), whereas the levels of hCG were 299-1600 mIU/ml. In 8 cases the progesterone levels were in the range of the secretory phase (2.3-6.9 ng/ml), and the hCG level was 182-5500 mIU/ml. In 1 case only was the progesterone level 15.0 ng/ml with an hCG level of 325 mIU/ml. In normal pregnancies of the same gestational age, the values of progesterone were 3.8-18.7 ng/ml, and the levels of hCG were 260-1300 mIU/ml. It seems that in addition to the level of hCG, a normal fetoplacental unit is needed for the preservation of the function of the corpus luteum.