The present study was aimed at evaluating magnesium content, ferric reducing antioxidant power (FRAP) and lipid peroxidation in selected tissues of rainbow trout during their development. For mineral and biochemical assay, samples of liver, kidney, gills and blood were taken from fish. Magnesium concentration ranged between 35.5 and 249.2 mg·kg⁻¹ wt/wt. Most magnesium was found in the gills and the less in kidneys. FRAP values in the examined fish varied from 0.85 to 4.64 nmol Trolox Eq.mg⁻¹ protein. The highest FRAP was observed in the kidneys and the lowest in the gills. Concentration of malondialdehyde (MDA) in the examined tissue homogenates averaged 4.16-11.36 nmol·mg⁻¹ protein. We observed that levels of analyzed parameters increased during growth of the fish.