Sodium fluxes in fresh and Na-rich uterine horns were studied. The change in efflux rate was compared with that in a theoretical model and analyzed by a curve-peeling technique. In both fresh and Na-rich tissues, the time course of efflux could be described by the sum of three exponentials. There was also a nearly inexchangeable ('bound') fraction of labelled sodium and a fraction of unlabelled sodium. Analysis of efflux led to the following equations:[Formula: see text]for fresh tissue, and[Formula: see text]for Na-rich tissues, where Y = tissue 22Na in millimoles per kilogram. The unlabelled part of the 'bound' Na amounted to 4 to 6 mmoles/kg in fresh and 6 to 8 mmoles/kg in Na-rich tissues. The amounts of 22Na in cells were calculated to be 7 and 34 mmoles/kg wet weight in fresh and Na-rich tissues. One phase of influx was similar to the fastest fraction of efflux and had a diffusion coefficient which was consistent with diffusion in the extracellular space (1/10 of that in free solution). The sodium content of this compartment was somewhat greater than sodium in the 14C-sucrose spaces (430 and 480 ml/kg in fresh and Na-rich tissue respectively). The second fastest sodium fraction (observed during efflux) was much larger in Na-rich tissues, and evidence that it contained cellular sodium was presented. Sodium efflux from cells was 5.9 pmoles cm−2 s−1 for fresh tissues and 26 pmoles cm−2 s−1 for Na-rich tissues. Comparable but less accurate values for influx were 4.2 and 7.0 pmoles cm−2 s−1. Assuming all emerging sodium was pumped against the electrochemical gradient, the energy expenditure would be 0.11 kcal/kg per h in fresh tissue and 0.22 kcal/kg per h in Na-rich tissues. It was concluded that this kind of active transport provides an unsatisfactory explanation for 22Na efflux.