Salinity is one of the important factors which affect drug metabolism in aquatic animals.After single oral dosing at 500 mg/kg praziquantel (PZQ) premix,we compared the tissue absorption and eliminations of PZQ in freshwater and brackish water cultured grass carp (Ctenopharyngodon idellus) by high performance liquid chromatography (HPLC).The results showed that salinity had a highly significant effect on concentration of PZQ in plasma,muscle,liver and kidney in grass carp (P<0.01).The peak concentration of PZQ in plasma,muscle,liver and kidney in brackish water grass carp were 0.98,1.1,3.99 and 3.2 μg/mL and in freshwater grass carp were 1.13,1.43,5.55 and 3.72 μg/mL,respectively.In both groups,it reached peak concentration at 1 h.The elimination half-lives (t1/2β) of all the tissues and plasma in freshwater grass carp were longer than that in brackish water grass carp.The elimination half-lives (t1/2β) of plasma,muscle,liver and kidney in brackish water grass carp were 10.77 h,5.57 h,3.49 h and 3.37 h and that in freshwater grass carp were 14.82 h,5.99 h,17.51 h and 5.36 h,respectively.These findings indicated that the osmotic pressure in the water environment may affect the tissue absorption and elimination of PZQ premix in grass carp,suggesting that the dosing strategy of PZQ premix should be adjusted by the water environment.