The effect of the nonselective beta blocker timolol on maximal cardiopulmonary exercise performance was evaluated in 28 men with previous myocardial infarction without effort angina (mean age 63 +/- 8 years). Patients were randomized to placebo or timolol (10 mg twice daily) for 4 weeks and then crossed over to the alternative therapy in a double-blind manner. At the completion of each treatment period, patients underwent symptom-limited maximal cardiopulmonary exercise on a cycle ergometer. Exercise time, heart rate, oxygen consumption (VO2), oxygen (O2) pulse and respiratory exchange ratio were measured at peak exercise and at a submaximal exercise level defined at a respiratory exchange ratio of 1.00. Timolol treatment reduced peak heart rate from 153 +/- 11 to 102 +/- 14 beats/min (-33%, p less than 0.001). Exercise time decreased from 680 +/- 91 to 633 +/- 78 seconds (-7%, p less than 0.001). Peak VO2 decreased from 25.3 +/- 4.7 to 21.4 +/- 3.5 ml/min/kg (-15%, p less than 0.001). O2 pulse increased from 12.9 +/- 1.9 to 16.7 +/- 2.3 ml/beat (+29%, p less than 0.001). Peak respiratory exchange ratio did not change significantly, indicating comparable effort. At submaximal exercise, defined at a respiratory exchange ratio of 1.00, there was no difference in exercise time between placebo and timolol. Heart rate decreased with timolol compared with placebo, from 126 +/- 16 beats/min by 31% (p less than 0.001), VO2 decreased from 18.5 +/- 4.3 ml/min/kg by 10% (p less than 0.001), O2 pulse increased from 11.5 +/- 2.0 ml/beat by 30% (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)