The effects of inhibitors of the sodium pump (K-free solution or ouabain) on Na and K fluxes, water content, and adenine nucleotides were studied in rat uterine horns. The same four-compartment model [Formula: see text] was applied to K efflux as to Na efflux. However, for K efflux the larger cellular fraction, C, contained 70% of tissue K, and D could usually be ignored. K-free solutions did not alter ATP, ADP, or AMP levels, and had only small effects on efflux of Na from fresh or Na-rich tissues. In these tissues ouabain decreased Na efflux from the faster, larger cellular fraction (BK2 ↓) without affecting adenine nucleotide levels. K-free solutions or ouabain increased sodium influx, but neither inhibitor caused swelling. The inhibitors decreased water content of fresh tissues and failed to interfere with extrusion of water accompanying Na from Na-rich tissues. In addition to the ouabain-sensitive pump, another Na pump insensitive to K levels or ouabain was postulated to be responsible for control of cell volume and part of Na efflux in uteri. The K efflux was increased by ouabain, but not by K-free solutions. The K influx was decreased by K-free solutions, but was not affected by ouabain. Increasing external K (concentrations of 18.4 mM and higher) did not diminish the effects of ouabain on Na or K efflux. Neither cocaine nor adrenaline prevented the effect of ouabain on the Na or K efflux or on the increased Na influx. No evidence was obtained that a Na exchange–diffusion system operated in K-free solutions. It was concluded that the accepted views about the operation of a Na pump involving (Na+ + K+)-activated, ouabain-sensitive ATPase have to be modified for rat uteri.