A double effect of Ca++ is observed on K+-absorption by Lemna minor: activation through coupled (probably metabolic) processes, and inhibition through a perturbation of the structure of the catalytical sites of K+-absorption. In contrast, Mg++, which also exhibits a structural inhibitory effect, has not apparently a metabolic actrvatory one. It seems that diffusive processes could account partly for the second phase of the dual-phase curves of absorption.