Total and selenium-dependent glutathione peroxidase activity, lipid peroxides and selenium concentration were determined in tissues of rats kept upon standard diet. The selenium concentration as calculated per mg of protein is as follows: liver greater than kidney greater than lung greater than heart greater than muscles greater than brain greater than erythrocytes. The lipid peroxide concentration as expressed by the malondialdehyde amount is a follows: brain greater than heart greater than muscles greater than lung greater than kidney greater than liver. In all the analyzed tissues, Se-GSH-Px activity was found as measured with t-butyl hydroperoxide as substrate, and total GSH-Px activity, as assayed with cumene hydroperoxide. A highly significant correlation was found between Se-GSH-Px activity and selenium concentration in selected tissues (r = 0.91, p less than 0.01), and an inverse linear correlation between lipid peroxides concentration and Se-GSH-Px activity (r = -0.75, p less than 0.1). Se-independent GSH-Px activity was estimated as the difference between total GSH-Px and Se-GSH-Px activity. In all the investigated rat tissues higher activity of Se-dependent GSH-Px than that of Se-independent GSH-Px was observed. In erythrocytes and muscles only the selenium dependent enzyme was detected. Also glutathione S-transferase activity was estimated in the above tissues. No GSH S-transferase activity was found in rat erythrocytes.