Na/K-ATPase can function as a signal transducer as well as an energy transducing ion pump. Cardiac glycosides (including ouabain and marinobufagin, MBG) are a new class of steroid hormones. Ouabain-activated signaling pathways lead to the induction of some early response proto-oncogenes, activation of transcription factors, and cardiac hypertrophy. Low concentration of ouabain also induced endocytosis of the Na/K-ATPase and compartmentalization of some signaling molecules (e.g. c-Src, EGFR, and p42/44 MAPKs) into clathrin-coated pits, early and late endosomes. Ouabain-induced endocytosis of the Na/K-ATPase depends on the activation of Src kinase, clathrin-coated pits formation, and caveolin-1 (the major component of caveolae). Moreover, low concentration ouabain significantly reduced transcellular Na⁺ transport. The data also show a stronge interplay of ouabain-induced endocytosis of the Na/K-ATPase and signaling transduction.