To evaluate changes in the liver ability to metabolize amino acids during intake of different proteins, the content of free amino acids was measured in the blood of the portal and liver veins of rats fed krill protein and casein. After 48 h of fasting the animals were given a suspension of krill protein and casein at a rate of 540 mg protein per 100 g bw. After 0.5, 1, 3 and 6 h the content of free amino acids was measured in the blood of the portal and liver veins by ion exchange chromatography with the use of an automatic amino acid analyzer. Administration of krill protein brought about a high enough (63%) absorption of amino acids by the liver during the 6-hour experiment. The degree of casein absorption was 31%. As compared to casein, intake of krill protein led to an earlier saturation of the body with amino acids having branched chains (P less than 0.05). The enzymatic systems of amino acid metabolism responded to the supply of the proteins under study in the same manner, however the rate of adaptation was different, amounting to 0.5 h for krill protein and to 3 h for casein. It is evident that the enzymatic systems involved in amino acid metabolism respond to the intake of krill protein to a larger extent than enzymes involved in metabolism of casein amino acids.