X-ray angiograms obtained from isolated perfused dog lungs were used to measure changes in the internal diameter of small intraparenchymal pulmonary arteries (150-1,600 microns) and veins (200-1,000 microns) in response to hypoxia or intra-arterial serotonin [5-hydroxytryptamine (5-HT)] infusion. The diameter changes in response to the two stimuli were measured over a range of stimulus-induced increases (delta Pa) in the total arteriovenous pressure drop. When the resulting delta Pa was small, all arteries in the diameter range studied constricted in response to either stimuli. The maximum decrease in diameter was approximately 25% with hypoxia and 36% with 5-HT. However, when delta Pa was large, arteries with a control diameter larger than approximately 800 microns distended with hypoxia. On the other hand, 5-HT constricted all the arteries in the size range studied regardless of the resulting magnitude of delta Pa. Hypoxia caused a small (approximately 9%) constriction in all veins in the diameter range studied independent of diameter or the magnitude of delta Pa, whereas in the concentration range studied 5-HT had no significant influence on these veins. An analysis of the potential impact of these vessels on total pulmonary vascular resistance suggested that although vessels in the size range studied contributed significantly to the total response to these two stimuli, vessels smaller than those studied also made a major contribution to the total response.