To evaluate the effect of human atrial natriuretic peptide (hANP)to pulmonary circulation, we calculated pulmonary artery compliance (Cpa) and pulmonary venous compliance (Cpv), using radionuclide angiocardiography and Swan-Ganz catheter. Cpa was calculated by Reuben’s equation using pulmonary arterial pressure records and pulmonary capillary wedge pressure (Ppcw). The pulmonary blood volume was measured with our own method, using radionuclide angiocardiography. Then, the pulmonary venous volume was measured by dividing pulmonary arterial volume from pulmonary blood volume. Cpv was calculated with pulmonary venous volume and Ppcw. Synthetic a—human atrial natriuretic polypeptide was infused in 22 patients with various heart disease. The infusion of hANP at rate of 0.1 μg/kg/min significantly decreased pulmonary arterial pressure, pulmonary capillary wedge pressure and right atrial pressure. The hANP infusion significantly increased cardiac output. The hANP infusion significantly decreased total systemic resistance and total pulmonary resistance. Although Cpa was not significantly changed, Cpv was increased significantly by the hANP infusion. The results suggest that human arterial natriuretic peptide have potent vasorelaxant effects in the pulmonary venous system.