With the acute toxicity test, we obtained the median lethal concentration (LC50) of naphthalene to the zebrafish for 96 h, which was 11.8 mg x L(-1). Then we set up five treatments(0, 1/6 LC50, 1/4 LC50, 1/3 LC50 and 1/2 LC50) and exposed zebrafish to these treatments for 0.5, 1, 2, 4, 7 and 14d respectively to study the effects of naphthalene on the antioxidant defense system in visceral mass of zebrafish. The results showed that GSH, GPx and GST were very sensitive to naphthalene and were inhibited or induced at 0.5 d exposure. The activities of GPx were induced on the whole after 0.5 d, but were inhibited at higher concentration treatments (1/3 LC50 and 1/2 LC50 treatments) at 14 d exposure; while the activities of GST and the contents of GSH were almost lower than that of the control. The activities of SOD were induced first and then inhibited after 2 d; the activities of CAT were inhibited almost in the all treatments after 1 d. The effects of naphthalene on the antioxidant defense system in visceral mass of zebrafish could be used as biomarkers to estimate the biological effect of fish exposed to polycyclic aromatic hydrocarbons.