Adenosine diphospate-5`-ribose (ADP-ribose), which is produced from NAD by the action of NAD nucleosidase, has dual effect on platelet aggregation in citrated platelet-rich rabbit plasma at concentrations more than 1 × 10^(-4) M, it induces platelet aggregation immediately on the one hand, and on the other hand it inhibits platelet aggregation induced by ADP and by collagen after the platelet that had aggregated in the presence of ADP-ribose dispersed again after a few minutes. Since the enzyme ADP-ribose pyrophosphohydrolase, which catalyzes the hydrolysis of ADPribose to AMP and ribose-5`-phosphate, could be identified in platelet-free plasma, it appears likely that the inhibitory effect of ADP-ribose on platelet-aggregation induced by ADP or collagen is due to AMP which is enzymatically produced from ADP-ribose in platelet-rich plasma.