The most obvious difference in the patterns of breathing in different mammalian species is that small animals breathe faster than large ones. While this may be related to the greater metabolic rate of small animals (in terms of body size) this can be only part of the explanation; for the normal breathing rate of a mouse is greater than a man can maintain for any period of time. Although the factors determining alveolar ventilation rate have received considerable attention, the mechanisms which control the frequency and depth of breathing are less clear. Otis, Fenn & Rahn (1950) have shown that for any given alveolar ventilation rate there is an optimal frequency of breathing, and that if a frequency greater or less than this is adopted (with corresponding adjustments of the tidal volume to maintain the same alveolar ventilation) the work of moving the lungs is greater. Christie (1953) has shown that the 'optimal rate' is adopted in several physiological and pathological conditions, and Mcllroy, Marshall & Christie (1954) have suggested that the pulmonary stretch receptors (mediating the Hering-Breuer inflation reflex) are responsible. Mead (1960), on the other hand, has presented results for guinea-pigs and human beings indicating that breathing rate and depth are adjusted to keep the average force of the respiratory muscles minimal, rather than the work performed on the lungs. Whichever view is correct, information on the control of respiratory rate and depth needs to be extended, and the controlling mechanisms worked out.We have assessed the mechanical properties of the lungs in a number of different species of experimental mammals to see if the values are consistent with the interspecific differences in breathing pattern. Apart from applying the results to the optimal rate hypothesis, the results are values for commonly used experimental animals which may be worth recording.

No abstract