Abstract: Gill ATPase activities of grass carp were measured by chemical method and the result were subject to single factor analysis of variance. Under normal condition, it was proved that the differences of ATPase of gills from different groups of fish caught at different seasons and acclimated under 16±4℃ for 3 to 7 days were not significant (p0.05). Then, tests of the effects of Hg2+, TCB (trichlorinated biphenyls) and LAS (linear alkyl benzylsulfonate) on ATPase of grass carp tissues In vitro were carried out. The results showed that Hg2+, TCB and LAS caused fairly strong inhibition to ATPase of grass carp gills. Satisfactory straight lines were obtained by plotting the probits of inhibition percentage of ATPase against the logarithmic concentration of toxicants. The concentrations required to effect 50% inhibition of Hg2+, TCB and LAS to ATPase of grass carp tissues were 1.8 ppm (kidney), 18.4 ppm (gill) and 3.9 ppm (gill), respectively. Enzyme tests also indicated that the sensitivity of kindey ATPase to Hg2+ was greater than that of the gill. The reliability of the enzyme preparation used for this experiment was verified on the level of reaction of enzyme to toxicants. The pattern of toxic action was summarized. And probable mechanism of toxicants on ATPase was also discussed. It was proposed that effects of toxicants on ATPase could be used as an index of toxicological assessment.