This study examined the response of 7-ethoxyresorufine-O-deethylase (EROD), glutathione-S-transferase (GST), glutathione peroxidase (GPx), glutathione content (GSH), level of thiobarbituric acid reactive compounds (TBARS) and circulating vitellogenin, in three-spined sticklebacks after 21 days of exposure to Diquat herbicide, commercial nonylphenol polyethoxylate (NPEO) adjuvant and mixture between Diquat and adjuvant. The results showed that adjuvant exerted more important oxidative effects than Diquat and that mixture effects were unlike to single additivity. This study argues for ecotoxicological risk assessment of adjuvants and mixtures of adjuvants and pesticides.