We administered ascitic fluid containing atrial natriuretic factor (ANF) monoclonal antibody to rats after 3 weeks of exposure to hypoxia while the rats were still hypoxic. In additional chronically hypoxic rats, we infused synthetic rat ANF. In conscious chronically instrumented rats, after a bolus dose of 5 μg i.v. ANF, pulmonary arterial pressure fell significantly from 26.5±2 to 21±2 mm Hg (p<0.01), reaching its nadir at 5 minutes without change of systemic arterial pressure, cardiac output, or heart rate. Pulmonary arterial pressure increased gradually from 26±4 to 34±4 mm Hg within 30 minutes (p<0.05) after acute administration of ANF monoclonal antibody and decreased transiently to return to baseline within 15 minutes after infusion of control ascitic fluid containing monoclonal antibody against an apolipopro-tein. Cardiac output and heart rate remained unchanged after both ANF monoclonal antibody and control ascitic fluid. In normoxic rats, acute administration of ANF monoclonal antibody did not cause significant changes in pulmonary arterial pressure, cardiac output, or heart rate. Rats receiving weekly intravenous injections of ANF monoclonal antibody that were started before initiation of exposure to hypoxia experienced significantly aggravated pulmonary hypertension and right ventricular hypertrophy compared with rats receiving repeated infusions of control ascitic fluid. However, there was no significant difference in small pulmonary arterial wall thickness or percentage of muscularized arteries at the alveolar duct level. These results suggest that endogenous ANF attenuates hypoxic pulmonary hypertension by decreasing pulmonary vascular tone.