The effects of once-daily therapy with doxazosin (1 to 8 mg/day) on exercise capacity, left ventricular performance and hemodynamics (radionuclide ventriculography) were compared with those of atenolol (50 to 100 mg/day) and placebo in a randomized, double-blind crossover trial in 16 patients (9 men) with mild hypertension. Both medications controlled blood pressure (BP) to a similar degree (mean BP was 150 ± 12, 137 ± 17 and 141 ± 14 mm Hg for placebo, atenolol and doxazosin, respectively) but by different mechanisms. Changes during maximal semierect bicycle exercise were similar to those seen at rest. Doxazosin decreased total peripheral resistance and maintained cardiac output, whereas atenolol decreased cardiac output. Exercise capacity (136 ± 56 watts with placebo) was maintained by doxazosin (135 ± 56 watts) but decreased with atenolol (122 ± 55 watts). Compared with atenolol, doxazosin slightly increased the left ventricular ejection fraction at rest and during exercise. The significance of this study is in the choice of a first-line antihypertensive agent. Both are once-a-day medications that control BP. However, doxazosin does so by improving the abnormal physiology of essential hypertension and, consequently, does not adversely affect exercise performance.