Abstract The plasma concentration of the purines xanthine, hypoxanthine, inosine, adenosine, and guanosine have been measured in plasma from DBA mice and normal human volunteers. The assay system employed is based on the fluorimetric detection of H 2O 2 produced via sequential catabolism of purine ribosides and purine bases to uric acid and uses commercially available enzyme preparations. Catabolic enzymes in vivo can rapidly alter the dynamic equilibrium of plasma purines once blood has been withdrawn; consequently, the use of inhibitors of purine catabolism in blood collection has major effects on purine recovery. The use of an adenosine deaminase inhibitor (erythro-9-(2-hydroxy-3-nonyl)adenine) during blood collection markedly influences purine profiles and ensures more accurate quantitation of in vivo plasma purine levels. The need for rapid separation of cellular elements after venepuncture is also demonstrated. In normal human plasma from five volunteers, average concentrations are hypoxanthine/xanthine, 0.7 μ m; inosine, 0.2 μ m; adenosine, 2.0 μ m; and guanosine, undetectable. The average purine levels from DBA mice are hypoxanthine/xanthine, 2.9 μ m; inosine, 2.3 μ m; adenosine, 6.1 μ m; and guanosine, undetectable.