Maximal exercise performance was evaluated in four adult foxhounds after right pneumonectomy (removal of 58% of lung) and compared with that in seven sham-operated control dogs 6 mo after surgery. Maximal O2 uptake (ml O2.min-1.kg-1) was 142.9 +/- 1.9 in the sham group and 123.0 +/- 3.8 in the pneumonectomy group, a reduction of 14% (P less than 0.001). Maximal stroke volume (ml/kg) was 2.59 +/- 0.10 in the sham group and 1.99 +/- 0.05 in the pneumonectomy group, a reduction of 23% (P less than 0.005). Lung diffusing capacity (DL(CO)) (ml.min-1.Torr-1.kg-1) reached 2.27 +/- 0.08 in the combined lungs of the sham group and 1.67 +/- 0.07 in the remaining lung of the pneumonectomy group (P less than 0.001). In the pneumonectomy group, DL(CO) of the left lung was 76% greater than that in the left lung of controls. Blood lactate concentration and hematocrit were significantly higher at exercise in the pneumonectomy group. We conclude that, in dogs after resection of 58% of lung, O2 uptake, cardiac output, stroke volume, and DL(CO) at maximal exercise were restricted. However, the magnitude of overall impairment was surprisingly small, indicating a remarkable ability to compensate for the loss of one lung. This compensation was achieved through the recruitment of reserves in DL(CO) in the remaining lung, the development of exercise-induced polycythemia, and the maintenance of a relatively large stroke volume in the face of an increased pulmonary vascular resistance.