The presence of Ni2+ in the incubation medium of maize root segments (Zen mays L. cv. Dekalb XL 640) induces an early and significant enhancement of the rate of proton extrusion both in the absence and presence of fusicoccin; with time, an inhibition of proton extrusion and a leakage of K+ appear. The inhibition of proton extrusion is accompanied by a decrease in the dark CO2 fixation and by a decrease in the level of malate in the cells. Ni2+ inhibits in vitro phosphoenolpyruvale carboxylase activity of maize roots. The data indicate a correlation between the operativity of the proton pump and that of the malate-pH stat mechanism for the homeostasis of cytoplasmic pH.