A kinetic study of the interactions in the uptake of NH4-, K-, and Na-ions by excised barley roots was performed using the continuous flow technique. In all cases the absorption of one ion was inhibited by the presence of a second ion, but the extent of the inhibition was limited. The presence of an inhibiting ion consistently resulted in an increase of the half-value of the uptake vs. concentration relation of the inhibited ion. The difficulties encountered in the interpretation of these results in terms of ion competition are discussed. On the other hand, it is shown that the characteristic features of the interaction are readily reconcilable with the concept of carrier competition.