Mice were treated with one of the potent progestagens, medroxyprogesterone acetate (MAP) or melengestrol acetate (MGA), at different stages of the oestrous cycle. They were tested for deciduoma induction, or for egg implantation, by oestradiol treatment combined with either injection of air into the uterine lumen or transfer of blastocysts to the uterus. The two important variables for both deciduoma induction and egg implantation were the MAP-oestradiol interval and the stage of the oestrous cycle during which the progestagens were injected. Maximal uterine sensitivity was obtained after the injection of MAP during oestrus with a 4-day MAP-oestradiol interval. Under these conditions, 34% of the transferred blastocysts developed to full-term viable fetuses. The injection of MAP or MGA did not usually induce pseudopregnancy as judged by the activation of cyclic CL; hence, the pregnancies were maintained by the activity of the injected progestagens.