The myocardial protective effect of captopril cardioplegia was investigated through the recovery of cardiac functions, biochemical changes and ultrastructural assays in the isolated working rabbit heart after hypothermic ischemic arrest for 3 hours. Hearts were randomly divided into 3 groups with 8 in each. Another 3 groups of hearts (8 in each) were arranged for studying the biochemical changes and ultrastructure during the ischemic period. In the control group (I), the hearts were protected by the modified St. Thomas' cardioplegic solution No. I, while in Group II and III the cardioplegic solutions contained 2.3 mumol/L and 23 mumol/L captopril respectively. The results showed that the percent recoveries of cardiac function in the 23 mumol/L captopril group compared with the control group were 97.9+/-9.8% vs 84.9+/-7.3% in left ventricular peak systolic pressure (P less than 0.01), 118+/-33% vs 79+/-20% in+ dp/dt max (P less than 0.01), 111+/-22% vs 75+/-18% in-dp/dt max (P less than 0.01), 145.6+/-44.8% vs 76.7+/-18% in coronary flow (P less than 0.01), 120.2+/-45.7% vs 36.4+/-22.2% in aortic flow (P less than 0.01), 127.4+/-27.3% vs 56.1+/-14% in cardiac output (P less than 0.01), and 118.2+/-33.5% vs 52.5+/-19.9% in stroke volume (P less than 0.01) respectively.(ABSTRACT TRUNCATED AT 250 WORDS)