In quiescent cat papillary muscles J(K), the rate of exchange of cellular K with K(42) in the steady state, has been measured in the presence and absence of NaCl over a wide range of temperatures. J(K) was found to be independent of the presence of external NaCl under the steady state conditions investigated. The Arrhenius plot for K exchange was linear over a range of temperatures from 2.5 to 37.5 degrees C in the absence of NaCl, and from 17.5 to 37.5 degrees C in the presence of NaCl. The corresponding apparent activation energies were, respectively, 10,200 and 8,800 calories/mole. J(K) in the absence of NaCl was not affected by 10(-5)M ouabain. These results are consistent with a passive distribution for the K of heart muscle cells. The observations suggest that a carrier-mediated forced exchange of K for Na does not occur during the steady state in mammalian heart muscle.