The pulmonary transvascular filtration changes were investigated in nine anaesthetized open-chest dogs during acute hypoxic exposure. The tracheobronchial pulmonary lymph flow, the lymph and plasma protein concentration, the pulmonary vascular pressures and the cardiac output were measured during three consecutive steady states of about 2 h each: 1) base-line, 2) hypoxia (FIO2 = 0.10), 3) recovery from hypoxia. It was found that pulmonary arterial pressure and lung lymph flow increased in all animals during hypoxia, respectively from (average +/- SD) 19.4 +/- 4.5 to 27.1 +/- 5.6 Torr, and from 27.2 +/- 10.2 to 59.8 +/- 23.8 microliters . min-1; lymph to plasma protein concentration ratio remained unchanged. During recovery, lung lymph flow remained elevated in some animals, while it returned to its initial value in the others. Control experiments with normoxic hyperventilation in five dogs showed no increase in lymph flow. The extravascular water/blood free dry lung ratio was moderately increased in animals that had been made hypoxic (4.64 +/- 0.67) compared to a control group (3.46 +/- 0.36). These results suggest that acute hypoxic exposure causes an increased pulmonary transvascular filtration in anaesthetized dogs.