The objective of this experiment was to investigate the effects of dietary casein supplement levels on the release of free amino acids (FAA) and peptide-bound amino acids (PAA) in plasma as well as the net flux in mesenteric-drained viscera (MDV) and portal-drained viscera (PDV) in Hu sheep. A 4×3 incomplete Latin square design was applied in this experiment. Three Hu sheep, which were fitted with portal vein, jugular vein and carotid artery catheters, were fed four diets containing different levels of casein as treatments: 0 (A), 3.0% (B), 6.0% (C) and 9.0% (D), respectively. The results showed that the concentration of amino acids in plasma increased in accordance with the increment of dietary protein levels. The net flux of FAA in MDV in treatment D was significantly greater than that in treatment A (P<0.05), and the net flux of PAA in MDV in treatment C was significantly greater than that in either treatment A or B (P<0.05). The highest net flux of FAA or PAA in PDV was observed in treatment C, which was significantly higher than that in either treatment A or B (P<0.05 or P<0.01）. These data imply that in certain extent, the net flux of FAA and PAA in either MDV or PDV of Hu sheep can be affected by dietary protein levels. ［Chinese Journal of Animal Nutrition, 2010，22（6）:1579-1584］