Effects of increasing dietary protein were studied in matrinxa, (Brycon cephalus), an omnivorous teleost from the Amazon Basin in Brazil. Four isocaloric diets were formulated to contain 20%, 27%, 34% and 41% of crude protein (CP). Plasma glucose levels were significantly increased while triacylglycerols were significantly reduced at 41% of CP. Free fatty acids were significantly reduced at each level of rising CP. Plasma amino acids and ammonium followed the dietary CP increase. Liver glycogen and amino acids were reduced; liver glucose and lactate were constant, and ammonium increased with the CP in diets. Muscle glycogen and pyruvate decreased, protein did not change, while lactate and free amino acids increased. Kidney glycogen proportionally rose with the increase of CP from 20-41%. Pyruvate and lactate augmented irregularly from 20-41% CP. A gluconeogenic profile was observed in the kidney; the liver worked as regulator of body glucose. Increase of dietary CP and decrease of carbohydrates pushed muscle and liver catabolism of fat and sugar stores to satisfy energetical demands. CP contents above 34% were not recommended for B. cephalus, if the carbohydrate does not compensate the metabolical demands.