Protein digestion and amino acid absorption along the intestine of the common carp (Cyprinus carpio L.), a stomachless fish : an in vivo study K. DABROWSKI Department of Basic Fishery Sciences Academy of Agriculture and Techno%gy 10-957 Olsztyn-Kortowo, Poland. Summary. Protein hydrolysis to peptides and free amino acids and the apparent absorption of amino acids (AAaa) were evaluated in different segments of carp intestine. The AAaa analysed using Cr z 0 3 as a marker indicated that 73.2 % of the amino acids were absorbed in the first 20 % of the intestinal tract and 5.3 and 21.5, respectively, in the following segments (20 % of gut length). Except for methionine and histidine, essential free amino acid concentration decreased significantly along the intestine. Of the non- essential amino acids, glutamate and aspartate concentrations increased in the hind gut. The absolute amount of the peptide amino acid fraction decreased towards the middle intestine but, expressed as a proportion of the total amino acid content, it changed little along the intestine : 49 and 54 % in the anterior and posterior intestine, respectively. The molar concentration of the peptide amino acid fraction was much higher in carp intestine (543.9 mM) than in rainbow trout (147.3 mM) or human (143.9 mM) intestine. Introduction. There is a relative paucity of data on the fate of ingested protein in the fish intestine. Lied and Solbakken (1984) investigated digestion and absorption of pro- tein in the cod digestive tract by determining the distribution of polypeptides of different molecular weights in the gut contents. Hydrolysis in the stomach of cod fed minced fish fillet led to a release of over 75 % of the peptides having molecu- lar weights of between 300 and 4 000. However, autohydrolysis was not taken into account in that study. In rainbow trout fed a casein-gelatin diet, peptide amino acids in the intestinal content of the pyloric caeca region. This decreased in (Dabrowski et al., 19861. Peptide amino acids constituted 80.3-89.0 % of the total amino acids in the intestinal content of the pyloric caeca region. This decreased in the mid-intestine and increased again in the rectum. Studies on the localization of protein digestion and absorption in stomachless fish (Stroband and van der Veen 1981 ; Dabrowski, 1983a) have not analysed hydrolysis in various parts of the intestine. Other works on the regional differen- tiation of the intestinal epithelium in cyprinid fish distinguished a proximal seg-