To evaluate the effect of oxygenation[95% O2+5% CO2] of St. Thomas Hospital No.2 cardioplegic solution[Plegisol], 20 isolated perfused rat hearts were studied under hyp-othermic[20oC] ischemic arrest for 2 hours with infusion of cardioplegic solution every 30 minutes throughout the ischemic period. Ten isolated hearts were studied with the oxygenated cardioplegic solution and 10 another isolated hearts with the nonoxygenated one. Mean oxygen tensions of the nonoxygenated and oxygenated cardioplegic solutions were 150mmHg and 470mmHg, respectively. Two in 10 hearts infused with the nonaxygenated cardioplegic solution were not recovered from nonworking heart due to persistent ventricular fibrillation. In comparing hem-odynamic parameters between both groups, the mean postischemic recovery[expressed as a percentage of its preischemic control value] was significantly greater with the oxygenated solution[in 10 recovered hearts] than the nonoxygenated solution[in 8 recovered hearts] [95.9$pm$1.8% compared with 88.5$pm$2.9% in peak aortic pressure, p<0.05, 75.7$pm$5.2% compared with 43.5$pm$6.5% in aortic flow, p<0.01, 75.5$pm$4.6% compared with 54.1$pm$5.6% in cardiac output, p<0.01, 78.3$pm$4.6% compared with 60.3$pm$4.6% compared with 60.3$pm$6.2% in stroke volume, p<0.05, and 80.4$pm$5.3% compared with 58.6$pm$7.0% in dP/dT, p<0.05]. It is concluded that oxygenation of St. Thomas Hospital No.2 cardioplegic solution improves cardiac electrical stability and postischemic hemodynamic recovery after ischemic arrest in the isolated perfused rat heart.