The binding of many ligands to cellular receptors induces a signaling cascade which generates inositol 1,4,5-trisphosphate (IP3). IP3 binding to its receptors in various internal compartments causes a rapid Ca2+ efflux into the cytosol. We now demonstrate that chloroquine blocks ligand-induced Ca2+ mobilization without affecting IP3 synthesis. The effect is independent of the ligand employed and occurred with five unrelated ligands; namely, alpha 2-macroglobulin-methylamine, angiotensin II, bradykinin, carbachol, and epidermal growth factor. Chloroquine, quinidine, and quinine, however, block binding of [3H]IP3 to its receptors by 90%, 88%, and 71%, respectively. These observations suggest a previously undetected mechanism by which these agents may in part function as antimalarials.