Control ofrhizosphere pHandexclusion ofAlbytheplasma membrane havebeenhypothesized aspossible mechanisms for Altolerance. Totestprimarily therhizosphere pHhypothesis, wheatcultivars (Triticum aestivum L.'Atlas 66'and'Scout'), which differ inAltolerance, weregrownineither complete nutrient solution, or0.6millimolar CaSO4, withandwithout AlatpH4.50. A microelectrode systemwasusedtosimultaneously measure rhizosphere pH,K",andH fluxes, andmembrane potentials (Em) along theroot atvarious distances fromtheroot apex. Incomplete nutrient solution, therhizosphere pHassociated withmature root cells (measured 10-40millimeters fromtherootapex)ofAl- tolerant 'Atlas 66'wasslightly higher thanthat ofthebulksolution, whereas roots ofAl-sensitive 'Scout' causedaverysmall de- creaseintherhizosphere pH.InCaSO4solution, nosignificant differences inrhizosphere pHwerefoundbetween wheatculti- vars, while differential Altolerance wasstill observed, indicating that therhizosphere pHassociated withmatureroottissue isnot directly involved inthemechanism(s) ofdifferential Altolerance. InAl-tolerant 'Atlas 66', growth inaCaSO4 solution with5micro- molar Al(pH4.50) hadlittle effect onnetK+influx, H efflux, and root-cell membranepotential measured incells ofmatureroot tissue (from 10-40 mm backfromapex). However, inAl-sensitive 'Scout', Altreatment caused adramatic inhibition ofK+influx and bothamoderate reduction ofH efflux anddepolarization ofthe membranepotential. Theseresults demonstrate thatincreased Altolerance inwheatisassociated withtheincreased ability of thetolerant plant tomaintain normal ionfluxes andmembrane potentials acrosstheplasmalemma ofrootcells inthepresence ofAl.