Ten healthy male volunteers performed maximal exercise tests on a bicycle ergometer. A control reading was obtained; at a second session propranolol 80 mg was given, and then placebo, 100 mg or 200 mg AQ-A 39 were administered in a randomised double-blind manner. At peak exercise, the heart rate averaged 191.6 beats/min during the control test, 185.8 beats/min after placebo, 172.4 beats/min with 100 mg of AQ-A 39 (-10%), 166.0 (beats/min) with 200 mg of AQ-A 39 (-13.4%); with 80 mg of propranolol, the heart rate averaged 132.8 beats/min and the workload accomplished was substantially lower than that accomplished with AQ-A 39 and placebo which remained the same as in the control test. No effect on blood pressure was observed. The rate pressure product was significantly reduced with AQ-A 39.