The aim of this study was to determine the effects of streptozotocin-induced diabetes on plasma reduced glutathione (GSH) and S-nitrosoglutathione (CSNO) levels. Further, the study investigated whether an antioxidant, pineal hormone melatonin, could protect against STZ-induced effects. STZ significantly decreased plasma GSH but increased the levels of plasma GSNO. Daily supplementation with melatonin restored plasma thiol to control values. Data suggest that STZ-induced hyperglycemia and compounds that act as scavengers of free radicals and peroxynitrite like melatonin may exert protection against STZ-incluced toxicity.