gamma-Irradiation is finding increasing use in the sterilization of pharmaceutical products. However, irradiation might also affect the performance of drug delivery systems. In this study, the influence of gamma-irradiation on the physicochemical properties of two commonly used non-steroidal anti-inflammatory drugs (NSAIDs) [naproxen sodium (NS) and diclofenac sodium (DS)] was investigated. The drugs were incorporated in poly(lactide-co-glycolide) (PLGA, 50:50; molecular weight 34 000 or 88 000 Da) microspheres. The biodegradable microspheres were irradiated at doses of 5, 15, 25 kGy using a Co-60 source. Drug loading of irradiated and non-irradiated microspheres with both 34 000 and 88 000 Da polymers were essentially the same. A significant difference was noticed in the particle sizes of the irradiated as compared to the non-irradiated formulations. Notably, in release studies, the amount of active substance released from PLGA microspheres showed an increase with increasing irradiation dose. In DSC, the glass transition temperatures (T-g) of microspheres exhibited a slow increase with irradiation dose. (C) 2002 Elsevier Science S.A. All rights reserved.