Maximum potassium uptake is achieved by beech mycorrhizas from concentrations of chloride of about 0.2 mM. Addition of NaCl up to 0.5 mM does not significantly reduce K+ uptake from 0.1 mM chloride solutions, although the KCl significantly reduces Na+ uptake. Potassium is rapidly lost from mycorrhizal tissues at temperatures above 20° C, but only after some delay from tissues at lower temperatures. Addition of calcium ions (up to 1.0 mM CaCl2) does not significantly affect potassium uptake or loss. CaCl2 added to RbCl solutions slightly reduces Rb+ uptake. Losses of K+ from mycorrhizal tissues at temperatures above 20°C may be prevented by the application of glucose or fructose. Neither the process of carbohydrate uptake nor the increased internal concentration of carbohydrate itself is responsible for the reduction in potassium loss. It seems probably that factors bringing about the retention of potassium are generated in carbohydrate metabolism. The potassium loss engendered by high osmotic pressure of the external solution seems to be a separate phenomenon from that which occurs at high temperatures.