Adenosine, added to blood, undergoes deamination in the red cells with the formation of inosine. The latter undergoes phosphorolysis to yield hypoxanthine and presumably ribose-1-phosphate. The pentose phosphate is metabolized to lactate yielding ATP in the process. The adenosine deaminase and the purine riboside phosphorylase of the red cell occur in the stroma-free hemolyzate. No adenine is formed from adenosine in the blood, nor is adenine metabolized by the erythrocyte. Xanthosine, guanosine, and inosine, added to blood, follow the same metabolic pathway as adenosine and their metabolism brings about a comparable degree of resynthesis of ATP and other organic phosphate esters and formation of lactate. The experimental evidence indicates that the phosphorolysis of the nucleoside is not the rate-limiting reaction in the metabolism of these compounds in the red blood cell.