The role of oxytocin (OT) in the ovine CL is not fully characterized. Oxytocin has displayed either luteolytic or luteotrophic effects depending on the protocol used and mode of administration. Oxytocin has been proposed to be a mediator of PGF2alpha-induced luteolysis. An experiment was designed to test the role of OT in PGF2alpha-induced luteolysis. Mid-ventral laparotomy was performed between days 7.5 to 9.0 of the estrous cycle (d 0 = estrus). To prepare ewes for surgery, 0.14 mg/kg of Ketamine (Columbus Serum, Columbus, OH; 100 mg/mL) and 0.3 mg/kg diazepam (Columbus Serum, Columbus, OH; 5 mg/mL) were injected i.v. and isoflurane (Columbus Serum, Columbus, OH) was used to maintain anesthesia during surgery. An osmotic mini-pump was sutured into the ovarian pedicle near the hilus of the ovary, and the catheter was inserted directly into the CL. In animals with multiple CL, one ovary was removed, or one CL was enucleated so as to leave only one CL. The osmotic mini-pump contained an OT receptor antagonist, Atosiban (ATO; Sigma, St. Louis, MO), in dilutions to deliver 0.25 (n = 3), 1.25 (n = 2), 2.5 (n = 3), or 5.0 (n = 3) µg/h or vehicle (70% Methanol: 30% Saline) in 0.5 µL/h. Forty-eight hours after surgery, 25 mg of PGF2alpha (Lutalyse Pfizer Animal Health, NY, NY) was injected i.m. (h = 0). Jugular blood samples (10 mL) were taken every 3 h beginning at h 0 and continued for 24 hours. At 24 h, ewes were euthanized, attachment of the catheter was verified, and the ovary was collected. Serum P4 in ewes treated with 2.5 or 5.0 µg/h ATO was lower than in vehicle-treated or control ewes at 0 h (P < 0.13 and P < 0.05 respectively). Content of P4 in CL was lower in PGF2alpha-treated than in control ewes (P < 0.04). Due to unexpected negative effects of ATO on P4 concentration, a second experiment was designed to determine if P4 concentration was reduced by ATO earlier in the estrous cycle, when the CL is insensitive to a single treatment with PGF2alpha. Osmotic mini-pumps delivering either 5.0 μg/h ATO (n = 4) or vehicle (n = 5) were implanted via mid-ventral laparotomy on day 3 of the estrous cycle. The same protocol as in experiment 1 was followed except that blood samples were collected every 8 h for 48 h after pump implant, and ATO was the only treatment. The pattern of mean serum concentrations of P4 during hours 16 through 48 was lower (P < 0.002) in ATO-treated than in vehicle-treated ewes (1.37, 1.04, 0.68, 0.84, 0.78, 0.67, and 0.8 ng/mL compared to 1.65, 0.95, 1.35, 1.18, 1.26, 1.36, and 1.56 ng/mL at 0, 8, 16, 24, 32, 40, and 48 h, respectively). Luteal weight and content of P4 in CL did not differ (P = 0.59 and P = 0.22, respectively). In conclusion, ATO (5.0 µg/h) decreased serum P4 concentration in ewes either during early or mid-cycle. Though P4 content in the CL was not decreased significantly (157.1 and 191.8 µg/g for treated and control ewes, respectively), more animals are needed to determine if luteal P4 was reduced. It is suggested that ATO reduced luteal function, perhaps by binding to an OT receptor type that ATO has been shown to stimulate in human myometrium. (poster)