Right heart overload is one of the most important prognostic factors in patients with chronic lung diseases. To clarify hemodynamic and therapeutic significance of atrial natriuretic peptide (ANP) in pulmonary circulation, plasma ANP concentrations were measured during right heart catheterization (RHC) in patients with chronic lung diseases under static and dynamic conditions. 1) Mixed venous, arterial and venous plasma ANP levels in 45 patients were significantly different one another (p less than 0.01). Significant relationships were also seen between ANP concentrations, and mean pulmonary arterial pressure (PPA; r = 0.85, p less than 0.01), right ventricular systolic pressure (r = 0.91, p less than 0.01), mean right atrial pressure (PRA; r = 0.48, p less than 0.01), and pulmonary vascular resistance (r = 0.84, p less than 0.01). 2) Arterial ANP levels were measured before and after 28% supplemental oxygen inspiration and administration of intravenous aminophylline (5 mg/kg) in patients with chronic obstructive pulmonary disease (COPD). Plasma ANP levels were significantly reduced concomitantly with the reduction of PRA after oxygen, and stroke volume, PRA, pulmonary capillary wedge pressure (Pcw) after aminophylline, respectively. 3) Plasma ANP levels were markedly elevated during both exercise with and without aminophylline (5 mg/kg), but the degree of elevation was significantly less with aminophylline. The reductions of PPA, Pcw and PRA were also observed during aminophylline exercise compared with control exercise, whereas plasma catecholamine levels increased similarly in both studies. These findings suggest that plasma ANP levels reflect the state of right heart load in patients with chronic lung diseases and that its levels are useful to evaluate therapeutic value as for right heart overload.