The effects of aprotinin, a protease inhibitor, on the ischemic and nonischemic isolated rat heart was investigated with the use of the modified Langendorff model. During phase I of the study, hearts were perfused with either low-dose aprotinin (105 KIU/L), high-dose aprotinin (106 KIU/L), or normal saline solution added to modified Krebs-Henseleit solution. No statistically significant differences in contraction amplitude, contractility, coronary flow, and wet/dry heart weight ratio were observed among the three groups of hearts. In phase II, hearts were exposed to a 40- minute period of global ischemia at 31° C. Ischemic arrest was induced by warm cardioplegia. Before ischemia and during cardioplegia, hearts were perfused with either aprotinin 106 KIU/L (n = 10) or normal saline solution (n = 10) for 30 minutes. On reperfusion, recovery of hearts treated with aprotinin was significantly better than that of control hearts, as reflected by better contractility (analysis of variance, p = 0.011), higher coronary flow (p < 0.025), and lower creatine kinase levels (p < 0.05). No statistically significant differences in contraction amplitude were observed between the two groups. When the effect of ischemia within each group of hearts was analyzed, the preserving effect of aprotinin was even more pronounced. In the control group, ischemia caused a decrease in contractility (p < 0.025) and a decrease in oxygen consumption (p = 0.006); by contrast, in the aprotinin group the preischemic values were maintained. Accordingly, we conclude that aprotinin at concentrations up to 106 KIU/L has no deleterious effect on normally perfused hearts and has a significant protective effect on the ischemic heart when used in high doses in the preischemic period.