By the compartmental analysis method of MacRobbie and Dainty, and Pitman, estimates of K(+), Na(+), and Cl(-) concentrations and fluxes were obtained for the cytoplasm and vacuole of coleoptile cells of oat, Avena sativa L. cv. Victory. Double labeling was used in experiments with (42)K plus (22)Na and with (42)K plus (36)Cl in a complete nutrient solution. At the plasmalemma, according to the Ussing-Teorell flux ratio equation, Na(+) is pumped out and Cl(-) is actively transported inward. The results with K(+) are less conclusive, but it is probably pumped in. At the tonoplast there is an active inward transport of Na(+) and probably of K(+), but the status of Cl(-) is uncertain, depending upon whether there is an electrical potential difference between the cytoplasm and vacuole. The results suggest that ion selectivity resides mostly in the plasmalemma. Possible errors in the estimates and interpretations are discussed.