The ability of the ischemic heart to release prostacyclin (PGI2) and thromboxane A2 (TXA2) was studied, together with the effects of these substances on the ischemic myocardium in open-chest dogs. We measured the plasma levels of 6-keto-PGF1 alpha and TXB2--which are stable metabolites of PGI2 and TXA2, respectively--as well as lactate and coronary venous blood flow. The dogs were divided into three groups of eight animals which received indomethacin (5 mg/kg), (E)-3-[4-l-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY-046) (1 mg/kg), or the vehicle. A transient increase in 6-keto-PGF1 alpha was observed in the great cardiac vein 5 min after the ligation of the left anterior descending coronary artery (LAD). TXB2 and lactate increased 30 and 15 min, respectively, after the ligation. Indomethacin prevented significant increases in 6-keto-PGF1 alpha and TXB2, but accelerated the lactate release. OKY-046 prevented significant increases in TXB2 and lactate release, but did not counteract the increase in 6-keto-PGF1 alpha. Although coronary venous flow decreased significantly 5 min after the ligation in every group, the flow returned to the preligation level 15 min after the ligation in the OKY-046 and the vehicle groups. Thus, we have demonstrated the release of PGI2 and TXA2 from the ischemic heart and suggest beneficial effects of PGI2 and of a selective inhibitor of thromboxane synthetase on the ischemic myocardium.