The Na+-K+--ATPase, or Na+ pump, is a member of the P-type ATPase superfamily. In addition to pumping ions, Na+-K+--ATPase is engaged in assembly of multiple protein complexes that transmit signals to different intracellular compartments. The signaling function of the enzyme appears to have been acquired through the evolutionary incorporation of many specific binding motifs that interact with proteins and ligands. In some cell types the signaling Na+ --ATPase and its protein partners are compartmentalized in coated pits (i.e., caveolae) the plasma membrane. Binding of ouabain to the signaling Na+-K+--ATPase activates the cytoplasmic tyrosine kinase Src, resulting in the formation of an active "binary receptor" that phosphorylates and assembles other proteins into different signaling modules. This in turn activates multiple protein kinase cascades including mitogen-activated protein kinases and protein kinase C isozymes in a cell-specific manner. It also increases mitochondrial production of reactive oxygen species (ROS)and regulates intracellular calcium concentration. Crosstalk among the activated pathways eventually results in changes in the expression of a number of genes. Although ouabain stimulates hypertrophic growth in cardiac myocytes and proliferation in smooth muscle cells, it also induces apoptosis in many malignant cells. Finally, the signaling function of the enzyme is also pivotal to ouabain-induced nongenomic effects on cardiac myocytes.