The role of prostaglandin F2 α (PGF2 α) in the oxytocic response to oxytocin (OT) was investigated by infusing OT directly into the uterine artery of the sheep and recording changes in both the pattern of uterine motility and the rate of secretion of PGF2 α at different stages of the estrous cycle. On Day 3 (estrus = Day 0), OT infused from 0.1 to 5.0 mU/min produced a dose-related increase in the rate of secretion of PGF2 α but increased intrauterine pressure (IUP) and the frequency of contractions only at intermediate (1 mU/min) or high (5 mU/min) doses. Although exogenous PGF2 α infused at 15 µg/h mimicked OT, the myometrium responded normally to OT when the production of endogenous PGF2 α was suppressed with indomethacin. On Day 8, an intermediate dose of OT changed neither the pattern of uterine motility nor the rate of secretion of PGF2 α. On Day 14, OT infused at rates as low as 0.025 mU/min increased the secretion of PGF2 α 4-fold but induced no change in IUP or frequency of contraction. At 0.1 mU/min, OT increased the rate of secretion of PGF2 α nearly 10-fold within 3 min but changed IUP only slightly and after a delay of 5 min. Even at higher doses, OT invariably elevated PGF2 α secretion but did not consistently raise IUP. We conclude that while increased contractile activity is not a sine qua non for OT-induced synthesis of PGF2 α, neither is increased synthesis of PGF2 α an essential intermediate step in the activation of the myometrium by OT.