The incorporation of 3H-amino acids by endometrial stromal cells was analyzed by radioautography during decidualization in the mouse. 3H-proline was administered to 5- and 6-day pregnant mice and to virgin mice in estrus used as controls. 3H-tryptophan was administered to 6-day pregnant mice. The animals were killed 30 min. after the injection of the radioactive label. The quantitative analysis of the radioautograms showed that the concentration of silver grains per unit area due to 3H-proline incorporation was higher in decidual and predecidual cells on the fifth and sixth day of pregnancy than in fibroblasts of virgin mice. Considering the pregnant endometrium by itself, the incorporation of both 3H-proline and 3H-tryptophan was higher in fully differentiated decidual cells and predecidual cells than in fibroblasts of the periphery of the endometrium and involuting decidual cells. These results show a clear correlation between different endometrial regions recognized by morphological criteria and their metabolic activity. The data also show that different cells of the pregnant endometrium have different metabolic activities, independently of the precursor that was used.