Serum levels of progesterone in hysterectomized gilts were determined to evaluate the effects of exogenous 17 beta-estradiol (17 beta-E2) on the secretory activity of aging corpora lutea. Gilts were hysterectomized on day 6 of the estrous cycle and given im injections of 17 beta-E2 to mimic blood levels of endogenous estrogen during normal pregnancy. Serum progesterone was determined every third or sixth day, and estrone and 17 beta-E2 were measured at 12-day intervals from days 42-192. Progesterone decreased (P less than 0.01) after day 104 in hysterectomized gilts given sesame oil. Exogenous estrogen consistently decreased (P less than 0.02) progesterone secretion during an extended period (days 45-108) in aging corpora lutea of hysterectomized gilts. Abrupt decreases in estrone, 17 beta-E2, and progesterone levels occurred after termination of estrogen injections on day 114. The decrease in the secretion of these steroids in hysterectomized gilts was similar to that observed in previous studies at parturition and during the early postpartum period. Estrogen treatment beyond day 114 inhibits follicular growth and suppresses ovulation, but behavioral estrus was induced during the terminal stages of luteal activity. In the absence of the uterus, aging corpora lutea respond to exogenous estrogen by decreased progesterone secretion, which is similar to that observed during normal pregnancy in the pig.