The Ussing-Theorell equation, which provides a fundamental test for the independent passive movement of ions under conditions of nonequilibrium, has been used to assess the active and passive components of K(+) uptake by segments of pea epicotyl (Pisum sativum L. cultivar Alaska), incubated for 24 hours in both 1-fold and 10-fold concentrations of a complete nutrient solution. Measurements of the rates at which (42)K diffused out of the segments provided data from which were estimated the K(+) content of, and the fluxes to and from, the nonfree space compartments, interpreted as being cytoplasm and vacuole. For this analysis the serial model of MacRobbie and Dainty and Pitman for the spatial arrangement of cell compartments was used. On the basis of these values, and measurements of electrical potential across the cell membranes, the vacuolar K(+) concentration was found to be fairly close to that expected as a result of passive diffusion between the cytoplasm and vacuole provided that no potential exists across the tonoplast. Cytoplasmic K(+) concentration, however, was much too high in both treatments to be accounted for in passive terms. It was concluded, therefore, that, on the basis of the model, the high ratio of influx to efflux was maintained in the cells by an active K(+) pump located at the plasmalemma. There is some reason to question the applicability of this model for flux analysis to the conditions of high net influx as encountered here; nonetheless, it provides a first approach to an over-all flux analysis in pea stem tissue.