The calcium-channel inhibiting agent, diltiazem, has been shown to enhance salvage of reperfused myocardium independent of effects on coronary blood flow or myocardial work. Because lipid peroxidation may be a mediator of reperfusion injury and modifiable by calcium-sensitive pathways, we evaluated the effects of diltiazem on the formation of malondialdehyde (MDA), a product of lipid peroxidation, in isolated rabbit hearts perfused with buffer under control conditions or after 60 minutes of ischemia with or without 3 minutes of reperfusion. Diltiazem (5xl0∼7 M) reduced tissue MDA content in seven reperfused hearts compared with levels measured in 14 hearts reperfused without drug (1.54 ± 1.09 [SD] compared with 3.57 ± 1.88 nmol/g, p<0.05). Superoxide dismutase and catalase were ineffective in reducing tissue MDA content in reperfused hearts (n=8; MDA concentration, 3.88 ± 2.82 nmol/g) although they were effective in preventing lipid peroxidation in separate studies in which oxygen-centered free radicals were generated directly by an infusion of xanthine oxidase and hypoxanthine. These results suggest that the salutary effects of diltiazem in the setting of reperfusion may be mediated by reduction of lipid peroxidation at a locus not accessible to scavengers of oxygen-centered free radicals or by a mechanism not mediated by free radical pathways.