SUMMARY: [3H]Thymidine autoradiography was used to study DNA synthesis in the uteri of spayed mice treated with progesterone and oestradiol. Progesterone suppressed DNA synthesis in the glandular epithelium whether oestrogen was given or not. It also suppressed DNA synthesis in the luminal epithelium. Here oestradiol produced morphological changes and eventual re-entry of some cells into DNA synthesis. Progesterone altered the nuclear morphology of the stromal cells and increased the number synthesizing DNA. In these conditions a single injection of oestrogen was followed 10–15 h later by the synchronized entry into DNA synthesis of 30–40% of stromal cells. However, a second injection produced no further response. It was concluded that progesterone stimulated stromal cells in the resting phase to enter the cell cycle and that oestrogen then accelerated their passage through a single round of replication and division by shortening the interval between mitosis and DNA synthesis, following which the cells withdrew from the cell cycle.