We studied the antioxidant effects of nine calcium antagonists (nisoldipine, benidipine, nilvadipine, felodipine, nicardipine, nitrendipine, nifedipine, verapamil, and diltiazem) by means of rat myocardial membrane lipid peroxidation with a nonenzymatic active oxygen-generating system (DHF/FeCl3-ADP). The order of antioxidant potency of these agents was nilvadipine > nisoldipine > felodipine > nicardipine > verapamil > benidipine. Their IC50 values (microM) were 25.1, 28.2, 42.0, 150.0, 266.1, and 420.0, respectively. In contrast, nitrendipine, nifedipine, and diltiazem had little inhibitory effect on lipid peroxidation. These six calcium antagonists could be divided into four types on the basis of their antioxidant mechanisms. Nilvadipine, nisoldipine, and verapamil, which showed antioxidant effects both before and after the addition of active oxygen, and reduced the dihydroxyfumarate (DHF) auto-oxidation rate, were chain-breaking and preventive antioxidants. Felodipine, which showed antioxidant effects both before and after exposure to active oxygen and increased the DHF auto-oxidation rate, was only a chain-breaking antioxidant. Nicardipine, which showed an antioxidant effect only before exposure to active oxygen and reduced the DHF auto-oxidation rate, was mainly a preventive antioxidant. Benidipine, which showed an antioxidant effect only before exposure to active oxygen and had no appreciable effect on the DHF auto-oxidation rate, could interrupt the chain reaction of lipid peroxidation at the initial step alone. Although these results suggest that the antioxidant properties of some calcium antagonists may be beneficial clinically in protecting against cellular damage caused by lipid peroxidation, further studies are required to establish the antioxidant effects of these agents in vivo.