In fish, metabolic changes and qualitative responses during different nutritional situations are highly controversial in the scientific literature, and for this reason the objective of this work has been to probe deeper into the adaptive behaviour of two important amino acid-metabolising enzymes, glutamate dehydrogenase (GDH) and alanine aminotransferase (AAT) of liver and kidney in trout. In the present study, we examined the long-term effects of endogenous or exogenous proteins--generated, respectively, by a prolonged starvation or by feeding a high-protein diet--on the kinetics of liver and kidney GDH and AAT. Feeding on a high-protein diet significantly increased the liver (100%) and kidney (49%) GDH Vmax and catalytic efficiency; the same kinetic parameters of AAT increased by 65% only in the liver enzyme, without changing the Km and activity ratio values. Starvation registered a significant increase of both enzymes, Vmax and catalytic efficiency in the liver, but activity was unaltered in the kidney. In addition, no significant changes were found in the Km or activity ratio. All enzyme kinetics showed a Michaelian behaviour without any evidence of sigmoidicity. The experimental results show strong adaptive responses in the kinetic behaviour of the enzymes of both tissues. With the exception of renal AAT, the remainder of the enzymes presented a marked influence in their kinetic parameters by an excess of protein. The results are discussed in terms of the possible adaptive role of enzyme kinetics to amino acid availability.