Alkylphenol polyethoxylates (APEs) are widely used as nonionic surfactants. Nonylphenol (NP), one of the derivatives of APEs, has been found in the aquatic environment in ranges from nanograms per liter to milligrams per liter. In this study, juvenile rainbow trout were exposed to 0 (control), 66, 220, or 660 mug NP/L for up to 28 days. Fish remained healthy under NP exposures of 0, 66, and 220 mug/L for the length of the experiment. All fish died after 4 days of exposure to 660 mug NP/L. Time-dependent NP bioaccumulation was detected in the tissues of fish exposed to 220 mug NP/L (P < 0.05) and histopathological changes were observed in the livers of fish exposed to 220 mug NP/L. Furthermore, an increase in the activity of glutathione-S-transferase (GST) was found in the liver of fish exposed to 220 mug NP/L for I week (P<0.05). There was an increase in GST activity in the liver of fish exposed to 66 mug NP/L but it did not occur before 2 weeks of exposure to NP. The GST activity then decreased in a time-dependent manner in treatment groups, and this decrease was lower in the livers of fish treated with 66 and 220 mug NP/L than in control fish after 3 weeks of exposure (P<0.05). These results indicated that sublethal doses of NP were accumulating in the bodies of the fish and causing histopathological. and biochemical changes in the livers of rainbow trout. (C) 2003 Elsevier Science (USA). All rights reserved.