The effects of 7-oxa-13-prostynoic acid (7-OPA), alone and as an antagonist of PGE<sub>2 </sub>and PGF<sub>2α</sub>, were investigated in isolated rabbit aortic and canine renal arterial (diameter ∼0.5 mm) strips. 7-OPA caused contractions in both preparations; threshold concentrations were 3-to 10-fold higher than the PGs and maximum contraction was 50-60%. In concentrations of 10<sup>–5</sup> M, 7-OPA inhibited contractions by PGF<sub>2α</sub> and PGE<sub>2</sub>. The same concentration of 7-OPA did not inhibit norepinephrine-induced contractions. These data indicate that 7-OPA is a partial agonist of PG receptors and produces its antagonism of PGE<sub>2</sub> and PGF<sub>2α</sub> by that mechanism.