The acute hypoxic pressor response was studied in 22 chronically catheterized awake rats, 13 in whom the pulmonary arterial circulation had been remodeled by 10 days of exposure to hypobaric hypoxia. Five of these had their hematocrit lowered to normocytic levels after the chronic hypoxic exposure. Nine were controls. After 24 h in room air the pulmonary arterial pressure (Ppa) and pulmonary vascular resistance (Rp) of hypoxic-polycythemic rats was at least twice the control value; in the hypoxic-normocytic rats Ppa and Rp were less than that of hypoxic-polycythemic animals and greater than that of controls. Cardiac index, heart rate, and O2 saturation were similar in all groups. In 10% O2 a rise in Ppa and Rp occurred in all groups; in absolute terms the rise was greater in hypoxic rats than in controls and greater in polycythemic than in normocytic animals. In the intact animal the acute hypoxic pressor response can still be elicited in a pulmonary vascular bed structurally altered by chronic hypoxia. When calculated as a percent increase over base line, its intensity was greater than in room air controls and for Ppa was independent of hematocrit.