The study was designed to investigate the special amount of peptides which could be completely absorbed in blood circulation of grass carp. Four groups of grass carp were infused into the front intestine with 6.25%casein (CA ), casein enzymatic hydrolysate (CSP), casein acidized (FAA ) solution and normal saline (1mg?100g -1 weight ) . Blood was collected after 20 min infusion from tail vein. The results of analyses by HPLC showed that the total amount of peptides and the amount of some peptides in plasma of grass carp given CSP were significantly higher than those of the grass carp given CA , FAA and normal saline ( P < 0. 05) . The total amount of peptides in plasma of grass carp given CSP was increased by 18. 73 % compared with those given FAA. There was no significant difference of total amount of peptides in plasma of grass carp given CA , FAA and normal saline ( P > 0. 05) . The increase of peptides in plasma was closely related with the kind and amount of peptides in intestine. The study indicated that small peptides could be absorbed into blood circulation of grass carp .