Intracellular Ca2+ is a regulator of active intestinal Na and Cl transport. Most studies have been done with rabbit ileum. Increasing intracellular Ca2+ decreases active Na and Cl absorption and/or stimulates active Cl secretion; lowering intracellular Ca2+ stimulates Na and Cl absorption. Based on studies with microvillus membrane vesicles from rabbit ileum, a direct effect of Ca2+ and calmodulin on linked Na and Cl uptake is established. Intracellular Ca2+ and cAMP affect the same transport processes and act in a nonadditive manner. Intracellular Ca2+ does not act by changing intestinal cAMP or cGMP contents, and increasing cAMP mobilizes intracellular Ca2+. Whether this Ca2+ is involved in regulation of ion transport is not known. The aspects of Ca2+ handling identified as involved in regulation of active intestinal Na and Cl transport include entry of Ca2+ across the basolateral membrane, mobilization of Ca2+ from intracellular stores, and involvement of the Ca2+-binding protein calmodulin. Several neurohumoral substances alter intestinal transport by Ca2+-dependent mechanisms and appear to act primarily by increasing (serotonin, carbachol, substance P, and neurotensin) or decreasing (dopamine) Ca2+ entry across the basolateral membrane of intestinal epithelial cells.