fed 0 mg/kg (control, CK), 15 mg/kg (low dose, LD), 30 mg/kg (mid dose, MD) and 60 mg/kg (high dose, HD) norfloxacin (NFLX) for 7 d.hange in gill and serum ethoxycoumarin-O-deethylase (ECOD), aminopyrine N-demethylase (APND), and glutathione-S-transferases (GST) activity at 2, 4, 6, 8, 10, 12, 24, or 48 h after the last oral administration=20 shrimp/tank) at salinity of 25.1±0.7, (22.0±1.0)°C and pH 8.1±0.6. At each time point we randomly selected eight shrimp from each replicate. The shrimp were immedi-Gill and serum GST activity was inhibited by NFLX. GST activity was 12.01 UmL after 4 h in the gill of the MD group and was only 8.87% the level in the CK group. Serum GST activity was 3.99-fold higher in the CK group than in the LD group after 24 h (<0.05). APND and ECOD decreased initially then increased and activity was significantly lower in all treatment groups at all times relative to the controls. Higher concentrations of NFLX were associated with increased inhibition of APND and ECOD.