The action of nitroprusside on pulmonary circulation was studied in patients with pulmonary hypertension due to mitral stenosis (group I) or to primary lung disease (group II) and in patients with normal pulmonary artery pressure (group III). Nitroprusside 20 microgram/min decreased systolic and mean pulmonary artery pressure in groups I and II while higher doses were necessary to produce a similar pressure reduction in group III. Diastolic pulmonary artery pressure was reduced by 40 microgram/min nitroprusside in all groups. Systemic arterial pressure declined during nitroprusside infusion similary in all groups in a dose-related manner; cardiac index remained unaltered. Pulmonary vascular resistance was reduced by 20 microgram/min nitroprusside in group I, not below 200 microgram/min nitroprusside in group II and remained unchanged in group III. Peripheral vascular resistance declined in all groups with nitroprusside infusion. The study suggests direct relaxation of the pulmonary vasculature by nitroprusside.