Ten dogs smoked cigarettes daily in two sessions each day by voluntary inhalation through a tracheostomy tube. Five dogs died during the course of the experiment, and the remaining five were killed after more than 420 days of smoking. Hematocrit values increased markedly during the first several weeks of smoking and then declined somewhat but remained higher than presmoking levels. The heart weight relative to body weight was markedly higher in the ten smoking dogs than in the ten control dogs. Pulmonary fibrosis and emphysema, similar to those conditions in human beings, were found in all five of the smoking dogs killed. No such lung parenchymal changes were found in ten control dogs.