The pulmonary function of 156 unanesthetized beagle dogs was measured to determine the influence of sex and aging. Equal numbers of males and females were compared at 12 to 14 mo. and 3 to 4 years of age. The larger body size of the males was reflected by higher values for tidal volume, minute volume, O2 uptake, lung volume, dynamic compliance, and diffusing capacity. The males had a more efficient ventilation and gas exchange than the females. Female dogs at ages of 12 to 14 mo., 3 to 4 years, and 8 to 10.5 years were compared and age-related differences were found which were similar to those reported for man. The older dogs had a higher frequency-lower tidal volume breathing pattern, an increased functional residual capacity, a reduced dynamic lung compliance, a reduced diffusing capacity, and a trend toward a lower arterial, Po2 and O2 saturation of hemoglobin. The similarities in the influences of sex and age on the pulmonary function of dogs and humans support the validity of extrapolating experimental data from pulmonary studies on dogs to man and demonstrate the potential usefulness of the dog as a model for studying lung aging.