Experiments were performed to evaluate the role of prostaglandin synthesis in the regulation of coronary blood flow in dog hearts. The left main coronary artery was cannulated and flow measured both in otherwise intact animals and in canine heart-lung preparations. Prostaglandin E was measured by radioimmunoassay. Reactive hyperemia (flow after occlusion release) was induced by coronary occlusion for 10, 15, and 20 s and was 39 plus or minus 13 (mean plus or minus SEM), 66 plus or minus 21, and 82 plus or minus 24 ml, respectively. Indomethacin, an inhibitor of prostaglandin synthetase, reduced reactive hyperemia at 10, 15, and 20 s to 15 plus or minus 5, 33 plus or minus 11, and 47 plus or minus 17 ml, respectively (P smaller than 0.05). Meclofenamate, a different prostaglandin synthetase inhibitor, gave similar results. In a second group of five dogs, prostaglandin production of the heart was examined in response to 20-s occlusions. There was a significant increase in prostaglandin production from a basal level of 18.6 plus or minus 4.9 mg/min to 35.3 plus or minus 5.8 ng/min after occlusion of the coronary artery for 20 s (P smaller than 0.05). After indomethacin, this increase in prostaglandin production was not observed and reactive hyperemia was significantly reduced. Thus, prostaglandin synthesis appears to be important to modulating canine coronary blood flow in response to brief periods of coronary occlusion.