Angiotension converting enzyme (ACE) inhibitors and beta-blockers have been reported to possess disparate effects on insulin sensitivity. The aim of this study was to study the effects of the selective beta-1 blocker bisoprolol and of the ACE inhibitor captopril on cellular insulin action in hypertensive individuals. After washout, 12 mild to moderate essential hypertensives were randomized in a double-blind manner to 5 mg bisoprolol daily or 25 mg captopril twice daily for 8 weeks. Erythrocyte insulin binding and insulin-stimulated tyrosine kinase (TK) activity were measured before and after therapy. Both agents decreased diastolic blood pressure significantly (bisoprolol 96.5+/-0.9 to 87.8+/-3.1 mm Hg; captopril 96.5+/-0.9 to 91.5+/-1.8 mm Hg; P < .05). Fasting plasma glucose, insulin, and insulin/glucose indices remained unchanged after both therapies, as did lipid profiles. Maximal insulin-stimulated TK activity, assessed by phosphorylation of the exogenous substrate poly-Glu80Tyr20, was significantly higher (P < .05) after bisoprolol treatment, but not after captopril treatment, when compared to placebo (bisoprolol 8.5+/-1.8; captopril 7.3+/-1.5; placebo: 6.4+/-1.3 pmol 32P-ATP/fmol bound insulin). However, captopril, but not bisoprolol, increased the sensitivity of the receptor TK activity, as measured by the half-maximal activity concentration (ED50). Specific insulin binding was not affected by these two agents. Thus, both captopril and bisoprolol may have favorable but different effects on TK activity and insulin action at the cellular level.