Progesterone and 17 beta-estradiol peripheral plasma levels have been determined during labor and after 1, 3, 6, 12, 24, 48, 72, 96, 120 and 144 h from delivery in a group of 7 women, whose corpus luteum had been removed at delivery and in a corresponding control group with intact corpus luteum. In both groups the results indicate a progressive fall of progesterone and 17 beta-estradiol, that is evident until 144 h for progesterone and 72 h from 17 beta-estradiol. The analysis of the two groups with the Student's t test has shown significantly lower levels in the group of women, whose corpus luteum had been removed, at the 6th h (p less than 0.01), 12th h (p less than 0.02) and 24th h (p less than 0.05) for progesterone and at 3rd h (p less than 0.01) and 6th (p less than 0.01) for 17 beta-estradiol. The significant difference for progesterone appearing later than for 17 beta-estradiol could be due to the fact that progesterone is also dismissed by the adipose tissue, where it is stored. A further statistical elaboration of the results, carried out by calculating the regression line and the transfer function, confirmed the different pattern in time of progesterone and 17 beta-estradiol plasma levels after delivery in the two groups of patients considered.