A densometric procedure, developed for studying the circulation time for small vol. of contrast medium injected into the vascular system, is descr. The technique was tested in a series of expts. in dogs, the aim being to ascertain the effect of occlusion of a pulmonary artery on the transit time through the pulmonary circulation. The transit time through the pulmonary circulation was shorter when one pulmonary artery was occluded than when there was unobstructed circulation through both lungs. This phenomenon would seem to constitute an appropriate compensatory hemodynamical change since it is essential that the minute vol. is maintained even when one lung is removed from circulation. The expts. provided indirect evidence suggesting that the pulmonary circulation adapts itself to occlusion of the pulmonary artery to one lung also through augmentation of the blood vol. circulating through the contralateral lung.