The effect of verapamil and dexverapamil on the development of liver preneoplastic foci was investigated in male Sprague-Dawley rats initiated with N-nitrosodiethylamine (200 mg/kg i.p.), fed on a diet containing 0.01% 2-acetylaminofluorene and subjected to partial hepatectomy, according to the hepatocarcinogenesis model developed by Solt and Farber. Administration of drinking water containing 0.03% verapamil or dexverapamil resulted in a decrease in the incidence and size of gamma-glutamyl transpeptidase-positive foci. The chemopreventive effect of dexverapamil was more marked than that of verapamil. These findings support the hypothesis that these two calcium channel blockers act by reducing the resistance of initiated hepatocytes to the mitoinhibitory and cytotoxic effects of 2-acetylaminofluorene.