Of 28 patients with total surgical correction of tetralogy of Fallot in adult life, 22 had previously undergone Blalock-Taussig shunt operation. Possible modifying effects of the shunt on pulmonary ventilation and perfusion and on exercise tolerance were investigated with spirometry, radiospirometry and bicycle ergometry. The observed ventilatory volumes and capacities and diffusing capacity were significantly lower than the predicted values. Vital capacity was significantly reduced in patients with adverse NYHA or haemodynamic classification. Between the various NYHA or haemodynamic groups no statistically significant difference was demonstrable in pulmonary function or diffusing capacity or in airway resistance. Ventilatory obstruction was found also in patients without previous shunt. Other lung function tests showed no significant difference between the shunt and the non-shunt group. Nor was distribution of lung perfusion significantly affected by shunt. No correlation was seen between lung function and bicycle ergometric findings. However, in non-shunt patients who had ventilatory obstruction or impaired diffusing capacity, exercise tolerance was significantly higher than in patients who had had palliative shunt.