When pseudopregnant hamsters were ovariectomized and their uteri traumatized on day 4, deciduomal growth (not significantly different from that seen in intact controls) could be maintained by daily injections of 1, 2 or 4 mg of progesterone either alone or in combination with 1, 2 or 5 y g of estrone. When ovariectomy was performed on day 1 of pseudopregnancy and uterine traumatization on day 4, induction and maintenance of a deciduomal response could be achieved with 1 or 2 mg/day of progesterone alone (or in some cases i n combination with 1 pg/day of estrone) from days 1 to 3 followed by 1 or 2 mg/day of progesterone in combination with 1, 2 or 5 Fg/day of estrone from days 4 to 7. The best results were obtained with progesterone alone ( 2 mg/day) before trauma and estrone ( 2 &day) and progesterone ( 2 mg/day) after trauma.