Changes in pulmonary circulation caused by muscular exercise and body position are usual in daily life. By using first-pass radiocardiography and fractal analysis, pulmonary circulation in man was evaluated at rest and during muscular exercise. At rest, pulmonary circulation was heterogeneous as described by the relative dispersion (which is the coefficient of variation, i.e. the standard deviation of the pulmonary transit times divided by the mean transit time; RD = 0·51 ± 0·06) and fractal in nature. During exercise, pulmonary circulation became more homogeneous (RD = 0·35 ± 0·04; P<0·001). The calculated fractal dimension decreases from 1·09 at rest to 1·06 during exercise. The model identifies a cubic-law response for circulation heterogeneity and a quarter-power law for resistance during muscular exercise.