Plasma levels of progesterone* were measured during normal human pregnancy by a sensitive and rapid competitive protein binding technique. During the first half of gestation 440 determinations in 321 women and in the second half of gestation, 209 determinations in 160 women were performed. The average plasma level of progesterone in the 5th week of gestation was 24.8 \ m=+-\7.3 (s) ng/ml which is above the normal range of the luteal plateau of the menstrual cycle (10\p=n-\20 ng/ml). Between the 5th and the 9th week of gestation the plasma concentration decreased significantly. * The following abbreviations and trivial names are used:Progesterone: pregn-4-ene-3,20-dione. 17a-hydroxyprogesterone: 17a-hydroxy-pregn-4-ene-3,20-dione. 20a-dihydroprogesterone: 20a-hydroxy-pregn-4-ene-3-one.Oestradiol: oestra-l,3,5(10)-triene-3,17/?-diol. Testosterone: 17/?-hydroxy-androst-4-en-3-one.HCG: human chorionic gonadotrophin. LH: luteinizing hormone.s: standard deviation, n: numbers of observations.