Protein digestion and proteolytic activity in the digestive tract of an omnivorous cyprinid.

1. In adult and juvenile roach (Rutilus rutilus) feeding on meal worms or grass and acclimated to temperatures between 6 and 20 degrees C the following variables were determined: pH, protein and proteolytic activity of gut fluid and faeces, food consumption, duration of gut passage and efficiency of protein assimilation. 2. Proteolytic enzymes of fish are very stable against autolysis but they disappear in posterior portions of the intestine, suggesting the existence of a pinocytotic process. 3. In herbivorous roach as well as in adult carnivorous roach feeding at 20 degrees C this results in very low proteolytic activities in the faeces, whereas in juvenile fish and in adults feeding on meal worms at lower temperatures, the process of reabsorption seems to be less efficient. 4. Daily production of proteases as well as "daily proteolytic duration" are higher in herbivorous than in carnivorous roach. 5. For the same amount of protein consumed, fish feeding on grass require 10 times higher proteolytic activities than fish feeding on meal worms.