The antianginal properties and the duration of action of two doses of felodipine, a dihydropyridine calcium antagonist with a vascular:myocardial potency ratio approximating 100:1, were investigated in 15 patients suffering from disabling effort angina pectoris with reproducible exercise tolerance. Felodipine (5 mg, 10 mg) and placebo were administered once in the morning on three different days, with a 24 h interval between them, according to a double-blind 3 x 3 latin square design, 5 times replicated. Symptom-limited cycloergometric exercise tests were performed 3 and 12 h after administration. Duration of exercise to ST segment depression of 1 mm and to peak exercise was increased (all P less than 0.01) by both doses of felodipine in comparison with placebo. Twelve hours after administration, the 10-mg dose induced a significant improvement in the exercise time and a smaller ST segment depression (all P less than 0.01) in comparison with the 5-mg dose. The relationship between ST segment depression and the pressure-rate product during exercise was favourably influenced by the 10-mg dose at 3 and 12 h after intake, and by the 5-mg dose only at 3 h after intake. These findings suggest an increase in coronary blood flow induced by felodipine. Apart from mild headache there were no other unwanted effects. In conclusion, felodipine improves exercise tolerance and reduces electrocardiographic ischaemia for up to 12 h after single oral administration in patients with effort angina. Increasing the dose from 5 mg to 10 mg produces a more prolonged effect, with increased exercise tolerance 12 h after intake.