The role of lipoprotein oxidation in promoting atherosclerosis is gaining recognition as its spectrum of effects is being unveiled. Accelerated atherosclerosis is a major cause of morbidity and mortality in diabetic patients. Treatment with ACE inhibitors reduces oxidation of low-density lipoprotein (LDL-ox) in hypertensive subjects, however, their effect on LDL-ox in diabetic patients is yet obscure. To evaluate the effect of the ACE inhibitor enalapril and the calcium channel blocker nifedipine on LDL oxidation in normotensive type 2 diabetic patients. A randomized single blinded cross-over study was conducted on 24 nonobese, metabolically stable, normotensive patients with type 2 diabetes who were randomly allocated to receive either enalapril, 10 mg/day, or nifedipine, 30 mg/day, for 4 weeks followed by a 2-week washout period. They were then crossed over to a 4-week course with the alternate drug. The oxidation of LDL was evaluated by three methods: dialdehyde analysis using the thiobarbituric acid reactive substances assay with and without the addition of CuSO4 as well as determination of conjugated dienes in the LDL lipid extract. The propensity of the serum to oxidize LDL was reduced by enalapril by 17-28% depending on the laboratory method used (P=0.0001). Treatment with nifedipine resulted in a rise in LDL-ox of 7-11% as compared to baseline (P<0.05). The difference between the effects of enalapril and nifedipine was statistically significant with all three laboratory methods used (P=0.0001). Both drugs were equally effective in reducing systolic and diastolic blood pressure without affecting HbA(1c) levels and lipid profile. The albumin excretion rate was significantly reduced during treatment with enalapril returning to baseline levels during the washout period and the nifedipine treatment course. Our findings suggest that oxidation of LDL is attenuated by ACE inhibition and augmented by some calcium channel blockers. This observation may contribute insight into the underlying mechanism of the therapeutic effects of ACE inhibition in diabetic patients. Copyright (C) 2000 Elsevier Science Ireland Ltd.