The effects on hepatic EROD (7-ethoxyresorufin O deethylase) in Mugil so-iuy exposed to benzo(a)pyrene (BaP), pyrene and their mixtures of equal concentration were investigated, at concentrations of 0.1, 1.0, 5.0, 10.0, 50.0 mug/dm(3), in experimental condition. Time-effects and dose-response of the biochemical indexs were observed. The results showed that the hepatic EROD activities were induced by the exposure of BaP. pyrene and their mixtures at high concentration. Dose-response connections were that the hepatic EROD activities were elevated with increasing concentration of the pollutants. The combined effect of BaP and pyrene at 1: 1 concentration ratio on hepatic EROD activity was antagonism.