The aim of this study was to examine the effects of Capsosiphon fulvescens extract (CFE) on levels of essential amino acids in serum and the rate of protein digestion. CFE contains approximately 68% carbohydrate as fiber, 17% crude ash, and 14% crude protein. Experimental rats were divided two groups, one group receiving a normal diet and the other a normal diet containing 5% CFE. To analyze the absorption of essential amino acids into the hepatic portal vein, blood from the hepatic portal vein was collected between 10 a.m. and 5 p.m. for 10 days. In vitro protein digestion was decreased in the group fed 5% CFE; these data indicated that CFE affected protease activity. We measured the absorption of essential amino acids in the serum of the hepatic portal vein, at 30-, 60-, 90-, and 120 minutes after feeding. Although there was no difference in the concentration of total essential amino acids between the two groups, the 5% CFE-fed animals had a decreased rate of absorption. Absorption of Lys and Thr into the hepatic portal vein was lower in the CFE-fed group than in the control group. The rate of absorption of Met was delayed nearly 50% in the CFE-fed group compared to the control group. On the other hand, the rate of absorption of Leu, Ile, and Val was increased; Phe showed the same. Therefore, we suggest that CFE could affect protein metabolism by increasing or decreasing the absorption rate of essential amino acids.