Parous ewes were injected intramuscularly with 250 or 750 µg of estradiol-17β per day for 2 successive days, beginning on alternate days of the estrous cycle from 1 through 11 (first day of estrus = day 0). Including controls, 94 ewes were used in the study. The injection of 250 or 750 µg of estradiol on days 1 and 2, 3 and 4, or 5 and 6 had no apparent effect on the weight or morphology of corpora lutes (CL). However, the injection of 750 µg of estradiol on days 9 and 10 reduced CL weights significantly by day 14, and the injection of either 250 or 750 µg on days 11 and 12 reduced CL weights significantly by day 15. The results indicate that injections of estradiol during the last week of the estrous cycle can initiate the precocious regression of corpora lutea.