1) The effect of calcium on the uptake of lithium by excised barley roots was investigated. The absorption of lithium was competitively inhibited by calcium. Strontium inhibited the uptake of lithium competitively, magnesium inhibited it uncompetitively. 2) No support was found for the hypothesis of a calcium imposed barrier which inhibits the absorption of lithium. 3) The Qio of the lithium absorption process between 20° C and 30° C was 2.3-2.5 whether calcium ions were present or not. 4) At 0° C a rapid initial uptake of lithium takes place followed by a slow absorption. The capacity for rapid uptake at 0° C may be regenerated by bathing the roots in water at 30° C for a short time. This capacity was not destroyed during incubation, even for prolonged periods, in water at 0° C. The rate of lithium uptake at 30° C was not influenced by the quantity of lithium that was previously absorbed at 0° C.