Observations are recorded of respiratory pressure and flow in the unanaesthetized dog, sheep, goat, cow, pig, donkey, mule, horse, chicken and duck. The pattern of air flow in and out of the lungs was measured with a flow transducer, which was attached to a mask retained on the head by a simple harness. Intra-oesophageal and intrapleural pressures were measured directly; intra-alveolar pressure was estimated by an interrupter technique. Species differences in the rate and depth of pulmonary ventilation were observed, but, with the exception of the Equidae, the patterns of air flow were similar. The full-grown donkey, mule and horses showed a double phase during expiration; this was not observed in the young animal. Although there was wide variation in body weight and pulmonary ventilation, the total swing in intrathoracic pressure during quiet breathing was of the same order of magnitude in all animals.