By the inj. of adequate doses of NaCN into the left branch of the pulmonary artery; the right branch of the pulmonary artery; the right ventricle, and the left ventricle, the circulation time from these sites of inj. to the carotid sinus was detd. in dogs with open chests, before and after division and closure of the left bronchus. From these results, the circulation time through the right and left lungs was calculated. When both lungs were aerated, there was no great difference in the circulation time through the right and left lungs. The avg. calculated circulation time for the right lung was 4.2 secs., and for the left lung, 4.6 secs. When 1 lung was rendered atelectatic bv the closure of its bronchus, the circulation time through the atelectatic lung was increased to an avg. value of 6.2 secs., whereas the circulation time through the aerated lung was decreased to an avg. of 2.6 secs.