A rapid method for measurement of ATPase activity was presented in this paper. Three methods of determination of inorganic phosphate be used to chemically measuring ATPase activity were compared. Homogenizing method of tissue, buffer constitutent adopted in experiment, and purification procedure of enzyme are were confirmed by experiment. The relationship between enzyme concentration and activity and the precision of this method were also studied. Gill and kidney Na~+ K~+ Mg~+ - ATPase activities of grass crap were measured by this method. And one way variance analysis was used in data handling. At normal condition, it was proved that differences of Na~+ K~+ Mg~(++)-ATPase of gill and kidney between fishes which were traped at different seasons and acclimated at certain temperature over three days were not significant (p < 0.05) Then, tests of effects of Hg~(2+), Cr~(6+), TCB (trichlo lorinated biphenyls) LAS(linear alkyl benzylsulfonate) on ATPases of grass crap tissues in vitro were carried out. The results showed that Hg~(2+), TCB and TAS had prominent inhibition on ATPase of grass crap tissues, Satisfied straight lines were obtained by pltting of percentage inhibition of ATPase on a probability axis vs. toxicants concentration on a log arthmic axis. The concentration required to yield 50% inhibition of Hg~(2+), TCB LAS to grass crap tissues were I. 8ppm(kidney), 18.44ppm(gill), and 3.92ppm(gill), respectively. The enzyme test also indicated that the sensitivity of kidney ATPase to Hg~(2+) was greater than that of gill to Hg~(2+). It was found that inactivation existed in effects of Hg~(2+), LAS on enzyme. The main action site of Hg~(2+) was Na~+ K~+ - ATPase. The method used in the experiment was verified on the level of reaction of enzyme to toxicants. The pattern of toxicants action were summarized. The significance of ATPase test was discuss It was proposed that effect of toxicants on ATPase could be used as an index of toxicological experiment. And the feasibility and perspective of this index were also discussed.