The effects on glucose metabolism by the beta-blocker atenolol and the angiotensin-converting enzyme (ACE)-inhibitor trandolapril were investigated in a randomised double-blind parallel group study of patients with primary hypertension. Twenty-six patients were treated with 50-100 mg atenolol and 27 patients with 2-4 mg trandolapril o.d. Intravenous glucose tolerance tests, euglycaemic hyperinsulinaemic clamps and serum lipid measurements were performed after 8 and 48 weeks of active treatment. After 48 weeks insulin sensitivity was reduced by 23% by atenolol while it remained unchanged during trandolapril treatment (+0.5%, P = 0.0010 for difference between treatments, ANCOVA). The effect on triglycerides (+22% vs -8.5%) and high-density lipoprotein cholesterol (-13% vs +0.7%) also differed significantly between atenolol and trandolapril. Results after 8 weeks were similar. Glucose tolerance was not affected by either drug. Atenolol reduced diastolic blood pressure (DBP) better than trandolapril (-15.3 mm Hg vs -6.6 mm Hg for supine DBP after 48 weeks, P = 0.012). The difference in effect on insulin sensitivity between the drugs corresponded to 25% of the baseline values of insulin sensitivity, and persisted over 48 weeks of treatment. The choice of antihypertensive treatment could influence the risk of diabetes associated with treated hypertension. Journal of Human Hypertension (2000) 14, 175-180.