OBJECTIVE--To assess the cardiorespiratory response to graded exercise in patients after the Senning operation for transposition of the great arteries. DESIGN--Cross sectional study. Exercise performance was assessed by determination of the ventilatory anaerobic threshold. This was defined as the exercise intensity at which the ventilatory equivalent for oxygen (VE/VO2) started to increase systematically without a concomitant increase in the ventilatory equivalent for carbon dioxide (VE/VCO2). SETTING--Outpatient department. PATIENTS--Sequential sample of 20 patients studied 5-10 years (mean 7.3 years) after the Senning operation. Age at exercise testing varied from 5 to 11 (mean 7.8) years. RESULTS--The ventilatory threshold in the patients was significantly lower than normal (p < 0.005) and averaged 79.3% (SD 13%) and 80% (12%) of the predicted normal value for age and weight, respectively. Also, the ventilatory threshold was surpassed sooner (mean 3.2 (range 2-5) minutes) in the patients than in normal controls (3.9 (2.6-4.6) minutes). The lower ventilatory threshold was associated with a subnormal oxygen uptake (ml/min/kg) during submaximal exercise, a higher than normal ventilatory equivalent for oxygen ratio, and a lower end tidal carbon dioxide tension in the patients compared with normal controls. In half of the patients heart rate was lower during graded exercise. This relative bradycardia was not associated with a normal ventilatory threshold, as often seen in fit young people. No major disturbances in rhythm were observed during exercise. CONCLUSION--Exercise performance, assessed by respiratory gas exchange, was slightly but significantly lower than normal in patients after the Senning operation. A subnormal ventilatory threshold, associated with a higher than normal ventilatory response during submaximal exercise, supports the hypothesis that oxygen delivery to tissues is slightly impaired in these patients.