Nifedipine has a vasodilatory effect on both the systemic and the pulmonary circulation. Its preferential effect on reducing the pulmonary vascular resistance has been studied in patients with primary pulmonary hypertension. In 4 patients with the Eisenmenger's syndrome complicating patency of the arterial duct (ductus arteriosus), we have studied the possibility of this selective pulmonary vasodilatory effect of nifedipine in reducing the right-to-left shunting. The degree of differential cyanosis was taken to reflect the right-to-left shunting. This was assessed continuously by 2 pulse oximeters applied to the right arm and leg. After sublingual nifedipine, the oxygen saturation of the right leg increased from pretreatment value of 79 +/- 5% (mean +/- SEM) to a maximum of 84 +/- 3% (P less than 0.01). Such a beneficial effect was maximal in the first 2 hours. On maintenance therapy, symptom-limited cycle ergometry showed increased exercise duration with comparable degrees of arterial desaturation and there was symptomatic improvement. This improvement disappeared on changing to placebo. It was concluded that nifedipine reduced right-to-left shunting and improved symptomatology.