Plasmalemma vesicles isolated from rice (Oryza sativa L.) shoots and roots were used to study the effect of different nickel and cadmium concentrations on the Mg²⁺-ATPase activity. In vivo application of the metals for 5 or 10 days resulted in a stimulation of the enzyme activity in shoots, while either increases or decreases were observed in roots depending on the concentration and the type of ion tested. In contrast, an inhibitory effect was always observed on shoot and root plasmalemma ATPase activity when these ions were applied in vitro. At the organ level, the content of Cd in plants treated with 0.5 mM metal was higher than that of Ni, especially in roots. No correlation between the metal ion levels in the plasmalemma vesicles and ATPase activties could be established. The possible mechanisms of heavy metal induced changes on ATPase activities are discussed.