Thirty ovariectomized sows were used in an experiment designed to determine whether the ability of the porcine uterus to release prostaglandin (PG) F_2α in response to oxytocin is regulated by progesterone (P₄) and estradiol (E₂). Sows were assigned to one of four treatment groups: 1) no steroids (ovariectomized controls; n = 8), 2) E₂ (n = 8), 3) P₄ (n = 7), or 4) E₂ P₄ (n = 7). P₄ and E₂ were administered so as to mimic the normal temporal changes that occur in these hormones during the estrous cycle. A group of intact sows (n = 9) was included for comparison. All sows received an injection of oxytocin (30 IU, i.v.) on Days 12, 15, and 18 postestrus. Jugular venous blood samples were collected from 60 min before through 120 min after injection of oxytocin for quantification of 13,14-dihydro-15-keto-PGF_2α (PGFM). Preinjection baseline concentrations of PGFM, the magnitude of the PGFM response above baseline, and area under the PGFM response curve (AUC) were calculated for each sow on each day and compared among treatment groups by ANOVA. Among the ovariectomized sows receiving steroid replacement, baseline concentrations of PGFM were low on Day 12 postestrus in all four groups. On Days 15 and 18, baseline concentrations remained low in the two groups that did not receive P₄ but increased in those that did. Both the magnitude of the response to oxytocin and AUC were small on Day 12 postestrus in all 4 groups. By Day 15, the magnitude of the response and AUC increased in the group that received both P₄ and E₂ but remained low in the other three groups. By Day 18, responses to oxytocin were greater in both groups that received P₄ than in those that did not. Baseline concentrations were similar in intact sows and in those that received both P₄ and E₂ on all three days examined. The magnitude of the response and the AUC were greater in the ovariectomized sows receiving P₄ and E₂ replacement than in the intact control sows on Days 15 and 18 postestrus. From these results, we conclude that P₄ and E₂ interact to control the time when the uterus begins to secrete PGF_2α in response to oxytocin and the amount of PGF_2α secreted.