The structure of the bronchial tree in the lung lobules has been studied and the number of lobules and acini in different lobes and in the entire lung has been determined in polar foxes and dogs. Branching pattern and the number of generations of interlobular and intralobular bronchioli have been defined. It has been found that the number of lung lobules in polar foxes was twice as high as in dogs, the lobule volume being smaller almost by half. A smaller number of acini in polar fox lobules enables the central regulation mechanisms to intensify the control of local ventilation. It broadens the possibilities for physiological responses to the influence of climatic factors of the Extreme North.