The authors measured the single-breath diffusing capacity for carbon monoxide (DLco), total lung capacity(TLC), functional residue capacity(FRC), and residual volume(RV) in anesthetized male hamsters, rats, guinea pigs, and rabbits whose weights varied from 40 to 3,500g. TLC (defined as an airway pressure of 25 cmH20) was calculated by neon dilution. The DLco was estimated by a modification of the single-breath method. There was a high correlation between body weight and the measurement of both the diffusing capacity and the lung volumes. No significant difference in DLco was observed in rats when measured in different body positions, at airway pressures of 10 or 20 cmH20, from FRC or RV, in male or female rats, or following hyperventilation.