We measured the saturated phosphatidylcholine in lung lavage fluid and in lung tissue after lavage in five vertebrate species. The amount of saturated phosphatidylcholine recovered by lung lavage and from lung tissue showed a direct log linear correlation with species alveolar surface area. The saturated phosphatidylcholine content of lung lavage fluid per square meter of alveolar surface area varied in the sequence: mouse greater than rat greater than rabbit greater than dog greater than cat, and showed a direct correlation with species respiratory rate. We compared the lavage (presumably mainly alveolar) and tissue saturated phosphatidylcholine with the theoretical minimum amount required to produce a monomolecular layer over an area equal to the computed alveolar surface area. The data suggest that there is an alveolar and a tissue reserve of saturated phosphatidylcholine. The size of the alveolar reserve varied in the sequence: mouse greater than rat greater than rabbit greater than dog greater than cat. We conclude that in each species studied there is an alveolar and tissue reserve of saturated phosphatidylcholine and that both reserves are larger in animals with rapid ventilatory rates and small alveoli than in animals with slower breathing rates and larger alveoli.