The pressure-flow relationship obtained by Agostoni and Fenn (1960) during maximal inspiratory efforts against various external resistances contains information from which inferences regarding the power output, and the internal impedance of the breathing system can be made. Similar inferences can be made from measurements obtained with submaximal but constant inspiratory efforts. An analysis is presented to reconcile the observed linear pressure-flow relationships with the usually observed hyperbolic force-velocity relationship of muscle.