Uterine segments of normal rats were incubated with [1,2-3H] progesterone in TC Medium Eagle HeLa for 20 min, 1, 2 and 3 h. Only one compound containing up to 98 % of the total extracted radioactivity was found in the extract of uterine segments and in the nutrient media at all the times studied. This compound was identified by analytical TLC, chemical reactions and crystallization to constant activity as progesterone. In vitro metabolism could not be observed under the applied conditions (10−7m) over a 3 h period, no radio-metabolites were detected in either the uterine segments or in the nutrient fluids.