The effects of exogenous oxytocin (OT) and estradiol-17β (E2) on plasma concentrations of prostaglandin (PG) E2 and 13, 14-dihydro-15-keto-PGF(2α) (PGFM) were investigated on Day 14-15 (NP) of the estrous cycle and Days 14-16 (PI) and 21-25 (EP) of pregnancy in the ewe. Basal concentrtions of PGFM were significantly elevated in utero-ovarian venous (UOV) plasma on Day 14 of pregnancy (4.05 ± 0.81 nM, mean ± SEM) compared to that observed on Day 14 of the cycle or Days 21-25 of pregnancy (2.29 ± 1.3 nM and 1.06 ± 0.56 nM, respectively). PGFM release increased significantly following intera-arterial bolus injections of 50, 500, and 5000 mU OT at 2-h intervals in all experimental groups. There was no significant difference in area and peak height of the PGFM response between the 3 groups studied. The time to peak PGFM response was, however, significantly longer in the PI group. No significant changes in concentration of PGFM were observed in any experimental group following 1-h infusions of E2 at 5, 50, and 500 pmol/min. Long-term (1-18 h) infusion of E2 at 83 pmol/min increased the peak height of the OT-induced PGFM response at both stages of gestation studied. PGE2 concentrations in UOV plasma were < 0.05 nM in all samples studied. These results demonstrate that PG release can be induced in response to OT during the period in which ovine trophoblastic protein-1 (oTP-1) is released by the conceptus. During pregnancy, oTP-1 does not appear to inhibit the E2 induction of uterine TO receptors.