The aim of this study was to analyze the antioxidative effect of osajin during prophylactic administration. The pathological model for in vivo experiment was the unilateral ischemia-reperfusion of kidney of the laboratory rat. The animals were randomly divided into five groups. Osajin was administrated orally in doses of 5, 10 and 20 mg/kg once a day to three premedicated groups. Placebo--0.5% solution of Avicel--was given to the fourth group and the fifth group was completely intact. The premedication lasted 15 days and subsequently the ischemia of the left kidney was incited in general anaesthesia for 60 min. The reperfusion lasted 10 min and it was finished by blood collection from the left ventricle and the reperfused kidney was recovered. Selected biochemical markers were assessed in blood: superoxide dismutase, glutathion peroxidase, total antioxidative capacity and malondialdehyde. The kidney tissue samples were used for histopathological examination. Laboratory and histopathological results confirmed supposed effects of osajine. The dependence between the effect and the applied dose of osajin was linear. The best biochemical results were reached after administration of osajin at the dose of 5 mg/kg. The best histopathological results were reached after administration of osajin at the dose of 10 mg/kg.