A study was undertaken to determine if normal healthy subjects can increase their endurance capacity consequent to endurance training during chronic beta-adrenergic blockade. Forty-seven subjects, 17 to 34 years of age, were randomly assigned to 1 of 3 treatments (placebo, propranolol, 160 mg/day, and atenolol, 100 mg/day) and then completed a 15-week aerobic exercise training program. All groups reduced their submaximal steady-state heart rates consequent to training; submaximal oxygen uptake was slightly reduced; submaximal stroke volume was increased only in the placebo and atenolol groups; submaximal cardiac output was generally lower; and arterial-mixed venous oxygen difference was increased after training in all 3 groups, suggesting decreased muscle blood flow and increased oxidative capacity. Maximal oxygen uptake and maximal treadmill time were increased in all 3 groups after training. However, while still on medication the atenolol group had significantly greater increases in maximal oxygen uptake and maximal treadmill time compared with the propranolol group. Because most patients will remain on medication, these results suggest a distinct advantage for cardioselective blocking agents. It is concluded that beta-adrenergic blockade does not reduce the ability of normal healthy subjects to gain the benefits associated with cardiorespiratory endurance training.