Phosphoadenosine diphosphate ribose(p-ADPR) produced from NADP by the hydrolytic cleavage of- membrane-bound NADPase is a specific inhibitor of NADP dehydrogenase but its metabolic fate has not bees clarified. Contrarx to our previous suggestion of the presence of phosphoadenosine diphosphate ribose phophosphohydrolase(p-ADPRase) p-ADPR seems to be metabolized to AMP and ribose-5phosphate(R-5-P) via ADPR by the combined action of alkaline phosphatase and ADPRase evidenced by the following results. 1. p-ADPRase can not be separated from ADPRase, a hydrolytic enzyme of ADPR to AMP and R-5-P, throughout the enzyme purification steps. 2. Hydrolytic cleavage of p-ADPR is strongly reduced in the presence of inorganic phosphate, an inhibitor of phosphatase, while that of ADPR is not affected. 3. Enzymatic production of inorganic phosphate from p-ADPR is greater than that of R-5-P. 4. Chromatographic results show that ADPR, AMP and R-5-P are produced from p-ADPR.