Cyclic ADP-ribose (cADPR) is a recently discovered cyclic nucleotide with Ca2+ mobilizing activity. Caged cADPR was synthesized by reacting cADPR with 2-nitrophenethyldiazoethane. Elemental analyses, 1H NMR, and extinction coefficient measurements indicate that the product contains only one caging group. Anion exchange high pressure liquid chromatography separated caged cADPR into two forms, which most likely represent isomers. Both forms could be uncaged with equal efficiency by UV exposure to regenerate cADPR. Photolysis of caged cADPR was accomplished effectively with a spectrofluorimeter. The efficiency of uncaging depended on wavelength with UV light shorter than about 320 nm being the most effective. Caged cADPR was biologically inactive and could induce Ca2+ release from sea urchin egg homogenates only after photolysis. Specificity of the Ca2+ release was shown by inhibition by 8-amino-cADPR, a specific antagonist of cADPR. To demonstrate its utility in live cells, caged cADPR was microinjected into sea urchin eggs. Photolysis using a mercury light source effectively regenerated cADPR and resulted in Ca2+ mobilization and activation of cortical exocytosis in the eggs.