The epithelial content of tracer Na can be calculated either from the exponential increase of unidirectional transepithelial Na tracer fluxes observed after addition of labeled Na to the solution bathing the luminal side of isolated epithelia or from the linear portion of the rate of accumulation of label in the serosal solution. The second method is not only simpler, because fewer data points are required and no logarithmic transformations are necessary, but also more accurate, as shown in experiments with stripped epithelia of rabbit descending colon. From the difference in tissue Na tracer content in the absence and presence of amiloride, which blocks luminal Na uptake, a Na concentration in the transport pool of 8-10 mM is obtained, assuming that all cells participate in active transport. From a comparison of the intracellular Na concentration derived from current-voltage relations of the apical Na entry step with the Na concentration in the epithelial Na transport pool, an estimate may be obtained of what fraction of the cell mass is involved in active Na transport.