An investigation of the urinary metabolites of the oral progestational agent dydrogesterone in healthy women of childbearing age is reported. The drug was administered in 3H-labelled form and the urine of the first 8 h, containing on average 38% of the radioactivity administered, was used as the source of the metabolites. It was fortified with urine collected during the first 8 h of a similar study with nonlabelled dydrogesterone. After enzymatic hydrolysis of conjugated metabolites, 43 different chemical species were isolated by means of extraction, followed by column and thin layer chromatography. Three of these metabolites, constituting about 70% of the urinary radioactivity, were positively identified as 20 alpha-hydroxy-9 beta, 10 alpha-pregna-4, 6-diene-3-one (52%), 21-hydroxy-9 beta, 10 alpha-pregna-4, 6-diene-3, 20-dione (18%) and 16 alpha-hydroxy-9 beta, 10 alpha-4, 6-diene-3, 20-dione (1%). Of the remainder, 20 (13%) were tentatively characterized as various products of oxidative attack, all probably having the 4, 6-diene-3-one configuration intact. It is concluded that the 4, 6-diene-3-one configuration is metabolically stable in combination with the 9 beta, 10 alpha configuration. This finding may explain why dydrogesterone, in contrast no progesterone, is orally effective, and lacks estrogenic properties.