Human CD38 is a novel multi-functional protein that acts not only as an antigen for B-lymphocyte activation, but also as an enzyme catalyzing the synthesis of a Ca²⁺ messenger molecule, cyclic ADP-ribose, from NAD⁺. It is well established that this novel Ca²⁺ signaling enzyme is responsible for regulating a wide range of physiological functions. Based on the crystal structure of the CD38/NAD⁺ complex, we synthesized a series of simplified N-substituted nicotinamide derivatives (Compound1–14). A number of these compounds exhibited moderate inhibition of the NAD⁺ utilizing activity of CD38, with Compound4 showing the highest potency. The crystal structure of CD38/Compound4 complex and computer simulation of Compound7 docking to CD38 show a significant role of the nicotinamide moiety and the distal aromatic group of the compounds for substrate recognition by the active site of CD38. Biologically, we showed that both Compounds4 and 7 effectively relaxed the agonist-induced contraction of muscle preparations from rats and guinea pigs. This study is a rational design of inhibitors for CD38 that exhibit important physiological effects, and can serve as a model for future drug development.