In a series of 11 dogs in which the left pulmonary artery was ligated, the blood from the occluded lung was absorbed by the remainder of the vascular bed and, consequently, there was no significant alteration in the pulmonary blood volume. This adaptation took place, apparently, with only slight modifications in the circulation time and the pressure of the system. Examination of the variables investigated suggests, to a certain extent, that the increased flow brings about the opening-up of new vascular channels.