Effects of progesterone on the function of corpus luteum in estrous cycle were compared between the first and the third estrous cycle after vaginal opening in rats. When 2.5 mg/100 g body weight (B.W.) progesterone was administered between 07:00 and 08:00 hr on the day of estrus or on day 1 of diestrus to rats showing the first estrous cycle, the diestrous period was persisted more than 9 days in 95% of animals. In the diestrous period, plasma prolactin (PRL) level showed daily changes and corpora lutea became functional. The luteal function was enhanced by the deciduoma formed after uterine traumatization on day 3 of diestrus. However, in rats showing the third estrous cycle, 7.5 mg/100 g B.W. progesterone was required to induce similar response to those obtained in the first estrous cycle. Also, no functional corpora lutea and no daily change in PRL secretion were observed in rats injected with progesterone on day 1 of diestrus in the third estrous cycle. These results indicate that progesterone acts more effectively for induction of pseudopregnant diestrus in the first estrous cycle than in the third estrous cycle.