Oxidative stress and antioxidant enzyme activities during the feeding periods of Oncorhynchus mykiss larvae maintained under routine fish-farming conditions were studied. This study examined the antioxidant response, specifically the main antioxidant enzymatic defences (SOD, CAT, GSHpx, GR and GST), of glutathione, and of lipid peroxidation, during the actively exogenous feeding of O. mykiss larvae at 7, 14, 21 and 28 days after the yolk-sac is exhausted. Protein content in the fed larval homogenates was determined by the Folin-phenol reagent method. Oxidative damage was measured by the formation of malondialdehyde (MDA) as an indication of lipid peroxidation. Fatty acid methyl esters were prepared from total lipid by acid-catalyzed transmethylation at 55 degrees C for 15 h. Changes in the fatty acid composition were analysed in a GC-17A Shimadzu gas chromatograph. Fat-soluble vitamins were also determined in the same period of O. mykiss. SOD, Catalase and Glutathione peroxidase activity sharply decreased in the 21 and 28 days fed larvae. GR activity was considerably lower than the other enzyme activities measured in 7 days fed larvae. Catalase, Glutathione peroxidase and GR showed their highest value in 14 days fed larvae over a period of 28 days feeding. Unlike other enzymes activities, GST activity showed its highest value in the final two weeks. SPUFA and C22: 6n-3 were found with a higher percentage in the fed larvae than in the feed. Fatty acid percentages of C18: 3n-3 and C20: 5n-3 in fed larval body were reflected by the higher dietary fatty acid percentages but, the percentage of C18: 2n-6 in fed larval body was not reflected. (C) 2016 Elsevier B.V. All rights reserved.