The aim of the work was to study the effect of rosmarinic acid (RA) on markers of oxidative stress in rats with diabetes. Diabetes was induced by streptozotocin (STZ), RA was administered orally for ten weeks. Water consumption was measured daily. Ten weeks after the first RA administration, urine was collected over 15 hours. N-acetyl-β-D-glucosaminidase (NAGA) activity, levels of thiobarbituric acid reactive substances (TBARS) and glutathione (GSH) were determined in the pancreas, kidney, and plasma. RA administration to diabetic rats ameliorated markers of oxidative stress, as well as water consumption and urination. We assume that RA may mitigate STZ-induced diabetic manifestations by protecting rat tissues against damaging effect of free radicals.