The administration of 500 μg PGF2α on day 5 of pseudopregnancy (day of uterine traumatization) caused luteolysis as well as lysis of the deciduoma between 72–96 h. When the PGF2α was postponed to day 6, 7 or 8 of pseudopregnancy instead of day 5, the regressive changes were found to be relatively less intense. Replacement of an exogenous prolactin (500 μg/rat) or hydroxyprogesterone caproate (1 mg/rat) for 4 consecutive days (day 5 to 8) consistently prevented the luteolytic effects of PGF2α. The possible mode of action of PGF2α is discussed.