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GTP catabolism induced by sodium azide or deoxyglucose was studied in purine nucleoside phosphorylase (PNP) deficient human B lymphoblastoid cells. In PNP deficient cells, as in control cells, guanylate was both dephosphorylated and deaminated but dephosphorylation was the major pathway. Only nucleosides were excreted during GTP catabolism by PNP deficient cells and the main product was guanosine. The level of nucleoside excretion was largely affected by intracellular orthophosphate (Pi) level. In contrast, normal cells excreted nucleosides only at low Pilevel while at high Pilevels, purine bases (guanine and hypoxanthine) were exclusively excreted. PNP deficiency had no effect on the extent of GMP deamination.