Since streptozotocin has been known to be chemically stable only under acidic condition(pH 4), the spontaneous degradation and cellular uptake of streprozotocin in neutral incubation medium was determined by chemical assay. Streptozotocin levels in both phosphate buffered saline (pH 7.4) and Krebs-Henseleit buffer (pH 7.4) decreased with a half life of about 2 hours. The presence of erythrocytes or pepatocytes under the same buffers did not affect the streptozotocin degradation rate at all. However, streptozotocin levels in plasma isolated from rats decreased rapidly compared to those in neutral buffers.