The effect of Ca on the uptake of the anion Br and the cations K (Rb) and Na was investigated, with emphasis on its relation to time. Absorption of the Br ion is stimulated in the presence of Ca ions. This stimulation is relatively stronger at low concentrations than at higher ones, and manifests itself without a lag-phase. The effect of the Ca ion on Br uptake is independent of temperature; it appears at 1.5° C as well as at 20° C. Moreover, this stimulation is more or less independent of the pH in the range between 3.8 and 7.0. Further, the stimulating effect on Br uptake is not specific for the Ca ion because other cations such as K, Na, and Li also increase the Br absorption albeit to a less extent than the divalent Ca ion. The view is advanced that cations screen the negative charges on the cell surface, as a result of which more absorption sites come within the reach of the Br ion. The effect of the Ca ion on Na uptake is manifested at a pH of 5.3 in the form of a reduction in the total amount of Na entering the roots. It was found that the Na uptake can be divided into two fractions, a steady-state Na uptake and an additional Na uptake taking place only over a limited period, viz. during the first two hours of the experimental time, and probably consisting of the adsorption of Na to cytoplasmatic sites. This additional Na uptake is entirely or partially eliminated by Ca, but, quite to the contrary, the steady-state Na uptake is not at all influenced by Ca. Evidence is advanced indicating that the effect of Ca on this additional Na uptake is based not on competition for negative adsorption sites in the cytoplasm but on a reduction of the permeability of the outer plasma membrane under the influence of the Ca ion, by which admittance to the cytoplasmic sites behind this membrane is denied to the Na ion. The influence of the Ca ion on the K uptake at pH 5.4 manifests itself as inhibitory, stimulatory, or neutral depending upon the K concentration and the experimental time. At low K concentration (0.01 m.e./l), analogous to the situation with Na, there is an additional K uptake of limited extent and duration which is inhibited by Ca. As in the case of Na, this additional K uptake is assumed to consist of an adsorption of K ions to negative adsorption sites situated behind a membrane, the permeability of which is decreased by Ca. The steady-state K uptake is in itself not Ca-sentitive. At higher K concentrations (0.2 m.e./l) the additional K uptake is no longer abolished by the presence of Ca. The fact that in the absence of Ca at higher K concentrations (0.2 m.e./l and higher) the uptake rate diminishes quite considerably after a few hours is to be seen as the result of an efflux of K coming into operation as the K content of the root increases. The results are discussed in relation to the interpretations presented in the literature with respect to the influence of the Ca ion on ion uptake. An attempt is made to localize the various physiological processes involved in cation uptake at the cellular level.