The CYP3A activity in different tissues of grass carp and the effect of fenvalerat on the hepatic CYP3A activity had been studied.There were three test concentrations,1.2,6.0,12.0 μg/L,respectively,a blank control,and a positive control of enrofloxacin to study the effect of fenvalerat on the hepatic CYP3A activtity in grass carp.The results showed that CYP3A activity was detected in liver,gill,kidney,spleen,intestine and brain,with the highest activity in liver.Fenvalerate significantly inhibited the hepatic CYP3A activity and this effect was enhanced with the exposure time.The effect was weaken after 3 days.There were no significant differences between the test groups.Therefore,the clinical use of this pyrethroid,especially combination therapy in aquaculture should be in caution.