The pulmonary pressor responses to prostaglandin F_2α, 5-hydroxytryptamine, angiotensin II, and alveolar hypoxia were significantly increased when blood flow was reduced from 10 to 5 ml/min in isolated perfused rat lungs. When blood flow was increased from 10 to 15 ml/min, the pressor responses to hypoxia and angiotensin II were reduced. We speculated that the changes in pulmonary vascular reactivity with alterations in blood flow were due to vessel distension and changes in vessel wall-to-lumen ratios and consequent effects on mechanical contractile efficiency