The excretion of pregnanediol, pregnanetriol and the 6-oxygenated metabolites of progesterone by fifteen women has been measured throughout pregnancy. During the first trimester the level of excretion of the 6-oxygenated metabolites varied from 0\m=.\4to 2\m=.\2 mg./day and at the end of pregnancy the range was from 3\m=.\5to 11\m=.\6 mg./day. During the last 6 weeks of pregnancy, pregnanediol excretion was relatively constant, whereas in seven of the patients the excretion of the 6-oxygenated metabolites continued to increase. Except at the end of pregnancy there was a close correlation between the 6-oxygenated metabolites and pregnanediol excretion. The results suggest that the 6-oxygenated metabolites and pregnanediol have the same precursor and that little, if any, of a 6-oxygenated progesterone is secreted during pregnancy.There appears to be little increase in pregnanetriol excretion during pregnancy; six of the subjects showed a significantly higher excretion in the third trimester compared with the first trimester of pregnancy, but the values were not outside the normal range for non-pregnant subjects. The specificity of the method of Fotherby & Love (1960) for the estimation of pregnanetriol was examined when applied to pregnancy urine. Between 1\m=.\1and 2\m=.\4 % of administered progesterone was excreted as the 6-oxygenated metabolites. In only one of five subjects was pregnanetriol excretion increased after progesterone administration.