ABSTRACT The radioactive substances obtained from subcellular uterine fractions and from plasma were separated and identified by analytical TLC's, chemical reactions and crystallization to constant activity 5 min, 20 min and 3 h after the injection of [1,2-3H]progesterone into oestradiol-primed castrated Wistar rats. It was found that the radioactive progesterone remained unchanged in all uterine fractions during these times. In plasma, metabolites were found 5 min, 20 min and 3 h after injection of labelled progesterone. In these studies radio-metabolites of progesterone could not be observed in the uterus of normal rats under the applied conditions, but only in the plasma.