The pressure-volume and pressure-flow characteristics of the excised lungs of 5 harbor porpoise were determined. It was found that: (1) in comparison to terrestrial mammals the lung volume relative to body weight is large, (2) the lungs are capable of emptying to less than 17% of total lung capacity, (3) peak expiratory flowrates of 5 to 10 vital capacities (VC).sec-1 are relatively high for mammals, (4) isovolume pressure flow curves showed a clustering of flowrate plateaus from 40 to 80% VC, with a marked drop in the flow rate plateau at 20% VC, (5) the profile of flow-volume curves is unusually square shaped with flows remaining above 2 VC.sec-1 at a lung volume of 20% VC, (6) this feature is responsible for the short emptying time of less than 0.2 sec. It is concluded that cartilaginous airway reinforcement, which extends to the alveolar sac, is responsible for the unusual flow-volume properties. These porpoises have the capability of exchanging a large percent of their lung gas very rapidly during their brief pass through the air-water interface.