Experiments were performed on conscious, chronically instrumented rats to assess the effect of chronic propranolol treatment on basal pulmonary hemodynamics and on hypoxic pulmonary vasoconstriction. Rats received either propranolol (4 mg/day) or vehicle for 31 days via implanted osmotic mini-pumps. Animals were then acutely exposed to 8% and 10% O2. Pulmonary arterial pressure (PAP), mean arterial blood pressure (MABP) and heart rate (HR) were monitored via chronically implanted catheters, and cardiac output (CO) was measured by thermodilution. Chronic administration o propranolol was associated with minimal changes in MABP, PAP and total pulmonary resistance (TPuR). However, HR and CO decreased and total systemic resistance (TSR) increased in propranolol treated rats. The acute hypoxic pulmonary vasoconstrictor response of propranolol treated rats was significantly less than that of untreated control animals. Systemic hemodynamic responses to acute hypoxia did not differ between the groups. These data indicate that chronic propranolol administration reduces the acute hypoxic pulmonary vasoconstrictor response in conscious rats, but does not alter basal pulmonary hemodynamics.