Technical Abstract: In plants, high capacity tonoplast cation/H(+) antiport is mediated in part by a family of CAX (cation exchanger) transporters. Functional association between CAX1 and CAX3 has previously been inferred; however, the nature of this interaction has not been established. Here we analyze the formation of "hetero-CAX" complexes and their transport properties. Co-expressing both CAX1 and CAX3 mediated lithium and salt tolerance in yeast, and these phenotypes could not be recapitulated by expression of deregulated versions of either transporter. Coincident expression of "Arabidopsis CAX1" and "CAX3" occurs during particular stress responses, flowering, and seedling growth. Analysis of "cax1", "cax3", and "cax1/3" seedlings demonstrated similar stress sensitivities. When plants expressed high levels of both CAXs, alterations in transport properties were evident that could not be recapitulated by high level expression of either transporter individually. "In planta" coimmunoprecipitation suggested that a protein-protein interaction occurred between CAX1 and CAX3. "In vivo" interaction between the CAX proteins was shown using a split ubiquitin yeast two-hybrid system and gel shift assays. These findings demonstrate cation exchange plasticity through hetero-CAX interactions.