In the past it has been generally assumed that pulmonary flow resistance (RL) increases with increasing flow and decreases with increasing lung volume. Recent work indicates that RL decreases with increasing flow, at least up to flow rates 3-4 times greater than those at rest, and increases progressively with increasing lung volume. This behaviour results mainly from dynamic pressure dissipations within the pulmonary tissues due to viscoelastic phenomena. In fact, during resting breathing the contribution of the latter to RL is more important than that of airway resistance.