Three experiments were conducted to evaluate the duration of activity of a long-acting progestin, medroxyprogesterone acetate: 1) induction of stromal mitosis in the endometrium; 2) implantation of blastocysts; and 3) inhibition of ovulation. In the 1st experiment, randomly bred albino mice were ovariectomized and injected with MPA 1 week later. They were injected at various intervals with 20 ng estradiol-17B in 0.05 ml arachis oil and killed 24 hours later. In the 2nd experiment, male and female mice were mated and the day of finding a vaginal plug was assigned day 1 of pregnancy. On day 4, both ovaries were removed and MPA was injected to maintain the state in which implantation can be initiated by injection of estradiol-17B. Successful initiation is shown by implantation swellings 72 hours later; absence of implantation suggests that the progestin is no longer present at an effective level. In the 3rd experiment, female mice were given MPA before they were mated with male mice. Ovulation suppresson was determined by the difference in time before parturition between MPA-treated mice and untreated mice. The results of all experiments were similar. Effective levels of hormone were observed for 6 days in most animals and maintained for up to 8 days in some animals. Some animals exhibited sufficient progestational activity to allow blastocyst implantation 12 days after injection, while stromal mitosis could not be induced after 8 days. It appears that the state of epithelium, rather than the stromal cells' ability to undergo hormone-induced proliferation, allows implantation as shown by the epithelial mitosis which was still suppressed at 10 days. 1 mg MPA induced progestin activity similar to the physiological state of early pregnancy. Mating was inhibited by higher doses of progestin for unduly long periods.