It is known that streptozotocin (STZ) penetrating into the organism generates nitrogen monoxide (NO). Therefore, it is justified to presume, that in beta-cell destruction thereby induced, peroxinitrit resulting from NO and superoxide (O2-) reaction has an important role. It has also been studied how pro- and antioxidant systems change in STZ induced experimental diabetes in rat organs. Beside pro- and antioxidant systems of plasma and red blood cell hemolysates, changes in homogenates of the following organs were studied: liver, kidney, heart, lungs, spleen, brain, muscles and pancreas. We tested and compared antioxidant enzymes (superoxide dismutase-, glutathione peroxidase- and catalase activities) glutathione reductase activity regenerate reduced glutathione (GSH). The oxidized, reduced glutathione values and lipid peroxidation changes were measured. From our studies it has appeared that STZ treatment generally induces an oxidative predominance in tissues. Changes in this model thereby, can be compared to changes occurring in type 1 human diabetic patients.