Abstract: Recent researches discover that the mucous membrane of gut renews frequently and it is the main tissue which consumes energy in viva.The energy used by the mucosa of terrestrial animal gut accounts for 20% of the total energy in the body and amino acids such as Glutamine rather than Glucose are the leading energy material utilized by intestines.Therefore,amino acids absorbed from daily ration (or from blood) are partly used for oxidative decomposition or protein synthesis by intestines itself.Thus,the utilization of essential amino acids from daily ration by the mucosa of gut maybe influence the full development of nutrition value of daily ration.For the moment,a few articles which state the absorption of amino acids by intestines of aquatic animals have been delivered,but the studies in which the absorption protein synthesis and liquid synthesis of amino acids by intestines are measured at the same time in identical conditions are few reported.The studies were conducted to determine protein and fat synthesized from L-Tyr osine and L-Praline by the intestines of C.auratus and Grass carp in vitro by using L.4,5-3HTyrosine and L-2,3,4,5-3HPraline as tracers and adopting the invitro perfusion technique.and then approach general laws about both amino acids absorption and utilization by aquatic animals intestines and also offer necessary theoretical basis for formulating ideal feedstuff of amino acids balance.In this experiment.the concentrations of L-Tyr are designed 0.5mmoL/L,1.5mmol/L and 2.5mmoL/L and the concentrations of L.Pro are 1.0 mmol/L,5.Ommol/L and 10.Ommol/L respectively,according to the respective solvency of the two different amino acids.Experimental amino acids are infused into the whole in-vitro gut of fishes,meantime,the flux was controlled at the invariable speed of 2.0mL/min by pump.At the 40min of perfusion,transmural amino acids across gut wall,flee animo acids retained in gut wall。protein-synthesized amino acids and fat-synthesized amino acids are meas-ured accurately. The results indicated that most amino acids retained in intestines compound fat and the others partly existed freely in guts,partly used for protein synthesis and 80 on.The percentage of amino acids used for fat synthesis was extremely markedly higher than that of amino acids used for protein synthesis in both C.auratus and Grass earp(pPPP<0.05).