The additional efficacy, duration of action and tolerability of felodipine were evaluated in patients with stable angina pectoris and a positive stress test who were already receiving therapy with a beta-adrenergic blocker. One hundred and twenty-eight patients were randomized to double-blind treatment with 5-10 mg felodipine once daily or matching placebo, and were evaluated by serial exercise testing during 12 weeks of treatment. Felodipine at 4 h significantly increased exercise duration assessed after 4 weeks of treatment (increase 34 +/- 65 s vs 18 +/- 71 s in placebo-treated patients; 95% confidence interval 1.01-1.11; P = 0.01), and after 12 weeks of treatment (increase 39 +/- 103 s vs 3 +/- 72 s; 95% confidence interval 1.01-1.16; P = 0.02). The time until onset of exercise-induced anginal pain and time until 1 mm ST depression assessed after 4 weeks of treatment also increased significantly with felodipine compared to placebo. No statistically significant changes in exercise test parameters evaluated 24 h after medication were observed. The addition of felodipine once daily demonstrated a sustained improvement in exercise duration in patients symptomatic despite treatment with a beta-blocker evaluated 4 h after drug intake. At 24 h post dose, no statistically significant effect was observed. Felodipine is well tolerated with a low incidence of side-effects and no adverse effect on quality of life.