Despite a remarkable gravity independent heterogeneity in both local pulmonary ventilation and perfusion, the two are closely correlated at rest and during exercise in the normal lung. These observations strongly indicate that there is a mechanism for coupling of the two so that local (V) over dot/(Q) over dot-ratio is kept fairly uniform throughout the lung. This is also necessary to achieve adequate gas exchange in the lung. it was recently suggested that oxygen-induced vasoconstriction has a slow and intense component that might contribute to the matching of ventilation and perfusion also under normal conditions (Vejlstrup and Dorrington 1993). We therefore simultaneously determined distribution of local (approximate to 1 1/2 cm(3) lung pieces) ventilation and perfusion in eight sheep a? normoxia (FiO(2) 21%) and after 10 min and 2 1/2 h exposure to hypoxia (FiO(2) 12%; four sheep) or hyperoxia (FiO(2) 40%: four sheep). We used a approximate to 1 mu m wet fluorescent aerosol and 15 mu m radioactive microspheres i.v. to measure local ventilation and perfusion, respectively. Neither hypoxia nor hyperoxia caused changes in the distribution of ventilation. After 10 min exposure to hypoxia or hyperoxia, distribution of perfusion was altered so that the correlation between values for local ventilation and perfusion decreased. After 2 1/2 h exposure to either hypoxia or hyperoxia, distributions of perfusion and (V) over dot/(Q) over dot-ratio had returned to baseline. These results show that distribution of perfusion is influenced by acute changes in oxygen tension, so that local matching of ventilation and perfusion is affected. Apparently, some mechanism restores the matching during extended exposure to the altered oxygen tension.