We have examined the interrelationships among transpulmonary pressure, flow, and volume during exhausting exercise in 10 normal adult males. Expiratory transpulmonary pressures during exercise were compared with flow-limiting pressures measured at rest by two techniques. In no case did pressures developed during exercise exceed to an appreciable extent the flow-limiting pressures. This indicates that, during near-maximal exercise, ventilation remains efficient as judged in terms of the pressure-volume relationships of the lung. The mechanical properties of the lung do not appear to limit ventilation during exhausting exercise in normal subjects. We could find no relationship between the magnitude of transpulmonary pressure and exercise limitation. There was no evidence that lung mechanics changed during exhausting exercise in normal subjects. The two methods for estimating expiratory flow-limiting pressures, the orifice technique and the isovolume pressure-flow method, gave similar results.