Deoxyinosine and deoxyguanosine are also excellent substrates for purine nucleoside phosphorylase. While the equilibrium for the reactions catalyzed by purine nucleoside phosphorylase favors nucleoside synthe sis, it would appear that in the cell the concentrations of the free purine and ribose 1-phosphate are too low to support nucleoside synthesis under most conditions. The main function of purine nucleoside phosphorylase in the cells seems to be its role in purine side degradation. This is supported by the conditions observed in the cases where a deficiency o purine nucleoside phosphorylase has been detected. Under these condi tions there is a large buildup of the substrates (inosine, guanosine, deoxy inosine and deoxyguanosine) for purine nucleoside phosphorylase with a corresponding decrease in uric acid formation.