The kinetics of NAD glycohydrolase activity in cardiac muscle homogenates were investigated using HPLC and TLC techniques to identify reaction products. NAD was found to undergo enzymatic hydrolysis yielding ADP-ribose (ADPR) as well as a cyclase reaction resulting in the formation of cyclic ADP-ribose (cADPR). Both ADPR and cADPR were further converted into adenine and ADPR, respectively. The kinetics of the formation reactions, that manifested a clearly distinguishable initial lag phase in the time course of cADPR formation, suggest that the hydrolytic reaction yielding ADPR and the cyclase reaction yielding cADPR probably represent alternative catalytic functions of the same enzyme, which might be controlled by endogenous ADP-ribosylation reactions.