Peripheral plasma levels of 17α-hydroxyprogesterone* were measured during normal human pregnancy. During the first trimester measurements of peripheral plasma progesterone were also included. Five cases of early pregnancies were followed with frequent blood-sampling until the 14th week of gestation. In one of the women daily blood samples were collected from 10 days prior to conception until the beginning of the 6th week. From the 18th week on, a total of 173 blood-samples were obtained from individual patients. During the 7th week the average plasma levels of 17α-hydroxyprogesterone reached a maximum of 3.7 ng per ml followed by a gradual decline. From week 14 to week 34 the mean level was around 0.5 ng per ml. Thereafter an increase towards term was noted with a mean plasma level of 17α-hydroxyprogesterone of about 2 ng per ml in week 40. Peripheral plasma progesterone decreased from a mean level of 27.0 ng per ml in week 5 to reach a nadir of 16.0 ng per ml in weeks 8 and 9 followed by a gradual increase. No correlation between the plasma levels of 17α-hydroxyprogesterone and the clinical outcome of pregnancy could be demonstrated.