The influence of the counter-ion on K uptake was investigated in excised barley roots. The effect of the counter-ion on K uptake is restricted to the first three to four hours of the experimental period. The results suggest that the counter-ion influences only the binding of K ions in the cytoplasm and not the accumulation to the vacuole; and fit into the previously developed general scheme of cation uptake, according to which the rate of vacuolar accumulation of K is limited at the tonoplast.