1. Dilevalol (R,R-labetalol) is a non-selective beta-adrenoceptor antagonist with beta 2-adrenoceptor agonist activity. Its effects after 1 month's administration on heart rate, blood pressure and muscle blood flow were studied in a double-blind crossover comparison with nifedipine in 16 hypertensive patients. 2. Dilevalol and nifedipine were similarly effective in lowering systolic and diastolic blood pressure at rest, but dilevalol limited the rise in systolic blood pressure induced by exercise more than nifedipine (rise of 27 vs 53 mm Hg respectively, P < 0.01). 3. Dilevalol decreased resting heart rate compared with nifedipine (73 vs 92 beats min-1 respectively, P < 0.01). Dilevalol limited the exercise induced rise in heart rate more than nifedipine (36 vs 48 beats min-1 respectively, P < 0.01). 4. Muscle blood flow (measured by strain gauge plethysmography) was not affected by either dilevalol or nifedipine at rest. After exercise, dilevalol caused an increase in excess blood flow compared with placebo (10.8 vs 5.1 ml min-1 dl-1 respectively, P < 0.01). The difference between dilevalol and nifedipine did not reach statistical significance (10.8 vs 6.5 ml min-1 dl-1 respectively, P > 0.05). 5. On blood pressure and heart rate, dilevalol demonstrated beta-adrenoceptor blocker activity at rest and on exercise. On muscle blood flow, dilevalol appeared to have no effect at rest, but may have acted as a beta-adrenoceptor blocker rather than as a beta 2-adrenoceptor agonist during exercise.