Eucapnic hyperventilation, breathing dry air, produces a two- to fivefold increase in airway blood flow in the dog. To determine whether airway blood flow responds similarly in the sheep we studied 16 anesthetized sheep. Seven sheep (1-7) were subjected to two 30-min periods of eucapnic hyperventilation breathing 1) warm humid air [100% relative humidity (rh)] followed by 2) warm dry air [0% rh] at 40 breaths/min. To determine whether there was a dose-response effect on blood flow of increasing levels of hyperventilation of dry air, another nine sheep (8-16) were subjected to four 30-min periods of eucapnic hyperventilation breathing warm humid O2 followed by warm dry O2 at 20 or 40 breaths/min in random sequence. Five minutes before the end of each period of hyperventilation, hemodynamics, blood gases, and tracheal mucosal temperature were measured, and tracheal and bronchial blood flows were determined by injection of 15- or 50-micron-diam radiolabeled microspheres. After the last measurements had been made, all sheep were killed, and the lungs and trachea were removed for determination of blood flow to trachea, bronchi, and parenchyma. In sheep 1-7, warm dry air hyperventilation at 40 breaths/min produced an increase in blood flow to trachea (7.6 +/- 3.5 to 17.0 +/- 6.2 ml/min, P less than 0.05) and bronchi (9.0 +/- 5.4 to 18.2 +/- 8.2 ml/min, P less than 0.05) but not to the parenchyma. When blood flow was compared with the two ventilatory rates (sheep 8-16), tracheal blood flow increased (9.1 +/- 3.3 to 18.2 +/- 6.1 ml/min, P less than 0.05) at a rate of 40 breaths/min but not at 20 breaths/min.(ABSTRACT TRUNCATED AT 250 WORDS)