CD38 is a type II glycoprotein that catalyzes the formation of cyclic ADP-ribose (cADPR), an intracellular calcium signalling molecule, from nicotinamide adenine dinucleotide (NAD+). Using a modified version of the fluorimetric cycling assay for cADPR which reduces between-subject variability, we report significant decreases in brain and lung cADPR, which although similar to previously published values, showed much less individual variation. The reduced variation within each group suggests that the range of cADPR is narrower than previously thought, and that the regulatory mechanisms controlling these levels are more finely tuned. We also report significant increases in brain, lung, and kidney NAD+ in the Cd38-/- mouse, and provide the first experimental demonstration of the proximate relationship between CD38 and NAD+. © 2006 Elsevier Inc. All rights reserved.