Considerable heterogeneity unrelated to the effect of gravity has been demonstrated for both local ventilation (V) and perfusion (Q) in the lung. Local ventilation and perfusion are well matched, so that the heterogeneity of the V/Q ratio is less than for ventilation or perfusion alone (Melsom et aL 1997). We are searching for the mechanisms responsible for the coordinate heterogeneity of ventilation and perfusion. Here, we ask how and to what extent physical exercise induces changes in the distribution of ventilation and perfusion. We measured local (approximately 1.5 cm3 tissue volume) pulmonary ventilation and perfusion simultaneously in six sheep before, during and after running on a treadmill. Local ventilation was determined from the deposition of labelled aerosol particles and local perfusion from trapping of radioactive microspheres. Cardiac output increased approximately 2.5-fold during exercise. V/Q-ratios were not normally distributed and we therefore present the heterogeneity as the interquartile range. At rest, the average interquartile ranges for local ventilation, perfusion and V/Q-ratio were 0.48, 0.51 and 0.39, respectively. During exercise, the corresponding values were 0.44, 0.40 and 0.32. Thus, the distribution of local V/Q-ratio was narrower than for ventilation and perfusion also during exercise. We found a moderate redistribution of relative flow towards the dorsal parts of the lungs when perfusion increased, but the increase in total perfusion and ventilation was for the most part throughout the lung. The results indicate that the coupling between local ventilation and perfusion is at least as potent during exercise as at rest. The correlation (r) between paired values in the two resting periods was 0.93 for ventilation and 0.91 for perfusion and thus indicates time stability for the two variables.