In the present study, direct evidence of hydroxyl radical production in livers of Carassius auratus exposed to pentachlorophenol (PCP) was provided using electron paramagnetic resonance (EPR) with spin-trapping. A dose-effect relationship was obtained between hydroxyl radical intensities and PCP exposure. It was observed that hydroxyl radical was significantly induced by 0.001 mg l(-1) (below the criteria for Chinese fishery water quality) of PCP exposure. A strong positive correlation (r=0.9581, p<0.001) was observed between PCP liver concentrations and hydroxyl radical intensities within 7d of PCP exposure, which suggests that hydroxyl radical are mainly produced from PCP itself. However, no correlation was observed between PCP liver concentrations and hydroxyl radical intensities after 7d, and a higher intensity of hydroxyl radical could still be observed when the PCP liver concentrations decreased to a lower level, which suggests that other mechanisms may possibly contribute to hydroxyl radical production after 7 d. The glutathione/oxidized glutathione (GSH/GSSG) ratio decreased below that of the control level during the entire period of PCP exposure (0.05 mg l(-1)), which suggested oxidative stress occurred. (C) 2009 Published by Elsevier B.V.