In 14 children who underwent a venous switch operation for complete transposition of the great arteries, exercise testing was performed 3 to 15 years (mean 8) after the operation. Exercise performance capacity was assessed by the determination of the ventilatory threshold during exercise and by the oxygen uptake (VO2) reached at a heart rate of 170 beats/min (VO2,170). The ventilatory threshold was defined as the highest oxygen uptake (VO2) at which the pulmonary ventilation (VE) stops to increase linearly when related to VO2. During exercise above this threshold a disproportionate increase in VE relative to VO2 is observed. The mean ventilatory threshold was significantly lower (p less than 0.01) than the normal mean value and averaged 72 +/- 15%, 67 +/- 15% and 70 +/- 13% of the predicted normal value for children of comparable age, weight and height, respectively. The mean value for VO2,170 also was significantly lower (p less than 0.05) than the normal mean value for children of comparable age, weight and height, and averaged 81 +/- 20%, 81 +/- 18% and 80 +/- 18%, respectively. Compared with normal control subjects of the same sex and age, the ventilatory threshold was surpassed sooner (p less than 0.001) and reached after 2 +/- 1 min instead of the 4 +/- 1 min required by the controls. In nearly half of the patients, a lower than normal (i.e. below the 95% confidence limit) heart rate response to exercise was observed. Theoretically, this could be interpreted as indicating a normal or high physical performance capacity.(ABSTRACT TRUNCATED AT 250 WORDS)