Dogs, cats, rabbits, guinea pigs and rats have been studied. The lung-thorax compliance per unit body weight and per unit vital capacity shows some significant differences between species. Static pressure required by a normal inspiration is, however, similar in different animals. The expiratory reserve per unit vital capacity and the functional residual capacity per unit total lung capacity are larger in the animals breathing at lower frequency. These findings are related to the hypothesis that animals breathing at lower frequency have a larger functional residual capacity in order to contain within small limits the changes in composition of the alveolar air due to the breathing cycle. Relationship between dynamic pressure and flow follows Rohrer's equation. The dynamic pressure required by a normal ventilation is similar in the different species. Relationship between rate of work of breathing and breathing frequency shows that the frequency typical of each animal is that corresponding to the minimum rate of work. Submitted on April 13, 1959