Guandu River is responsible for providing drinking water for more than 10 million people from Rio de Janeiro metropolitan region. However, several studies have shown that this river presents high levels of substances which may cause mutations and cancer. Therefore, it is extremely important measurements be taken to improve this watershed water quality. This study aimed evaluating the potential use of ethoxyresorufi n Odeethylase (EROD) hepatic enzyme from tilapia (Oreochromis niloticus) as a

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... marker for monitoring the Guandu River water quality. Enzyme activity was determined by fl uorescence method in tilapias microsomal and post-mitochondrial fractions captured in the Guandu River and compared to two control sites. Our results showed that EROD activity was 700% higher in the Guandu River tilapias than those ones at control sites. This high EROD activity in tilapia livers from the Guandu River indicates large induction of CYP1A subfamily. Several authors have demonstrated that cytochrome P-450 isoform induction is associated to contaminants PCBs and dioxins presence, what present carcinogenic activities. Based on these results, we do not recommend fi sh consumption caught in the Guandu River and suggest further research must be done so these environmental contaminants origin could be clarifi ed.