Progesterone -6,7-3H (P*) was incubated in minces of human secretory and proliferative endometrium in absence as well as in presence of 10 and 100 micrograms/ml of unlabelled progesterone (P), in Eagle's Culture Medium throughout 72h. The following metabolites of P* were found in culture media: 1. C-21 derivatives of P* reduced at C-5, and C-20 positions. Also, a 3 beta-hydroxy-5 alpha pregnane-20-One conjugated to a glucuronic acid moiety was identified. 2. Concentrations of water-soluble derivative accounted for 21% and 29% of the recovered radioactivity in proliferative and secretory endometria, respectively. 3. After beta-glucuronidase cleavage, 80% to 95% of the water soluble derivatives were released as free steroids. 4. Approximately 60% corresponded to 3 beta -hydroxy- 5 alpha pregnane-20-one of the pooled water extracts. 5. Alson, 17% and 13% as well as 9% and 8% recoveries under 10 and 100 micrograms/P were observed in proliferative and secretory endometria, respectively. Glucuronidation seems to be a compensatory route to metabolize P and P* in human endometrium as might also occur in other species.