The aim of this study was (1) to assess the effects of dietary protein content and feeding level on trophic shifts of C and N isotopes (Delta delta(13)C(tissue-diet) and Delta delta(15)N(tissue-diet)) and (2) to test whether the measurement of the activities of two enzymes involved in the metabolism of amino acids could improve the accuracy of estimation of the trophic shifts of C and N isotopes. For this, 36 Nile tilapia (Oreochromis niloticus) were kept under controlled conditions for 8 weeks and fed at three different levels (2, 4 and 8 g kg(-0.8) d(-1)) with three diets differing in their protein content only (20, 29 and 39 %). For each fish, food to fish body trophic shifts of C and N isotopes were measured as well as the hepatic activities of aspartate aminotransferase (ASAT) and glutamate dehydrogenase (GDH). The feeding level affected the activities of ASAT and GDH as well as the trophic shifts of C and N isotopes significantly but the dietary protein content had no significant effect except on the specific activity of ASAT. Fish fed at the lowest level had significantly higher trophic shifts of C and N isotopes than fish fed at higher levels. The trophic shifts were significantly lower in fish with a high protein utilisation. Values of the 'goodness-of-fit' for linear regressions between enzyme activities and trophic shifts were low. Thus, activities of ASAT and GDH are not suitable for predicting estimates of trophic shifts in situations where the amount of food consumed or the dietary protein content is not known. In further studies, activities of enzymes involved in the metabolism of amino acids combined with measurements of the activities of other enzymes should be used to try and improve the accuracy of estimates of trophic shifts.