A detailed kinetic model is being constructed of the action of (Na,K)-ATPase, incorporating as much information as is presently available, with the object of establishing whether an extensive range of linear behavior can be found. Such behavior is characteristic of the sodium pump in epithelia such as frog skin, toad skin, and toad urinary bladder, and is also characteristic of the proton pump in turtle urinary bladder. Moreover, linear behavior is observed in oxidative phosphorylation by mitochondria. Linearity of this type has important implications in the nonequilibrium thermodynamic analysis of the efficiency of energy conversion, degree of coupling, and stability of biological energy transducing systems.