l-carnitine, a multiphysiological, bioactive and pollution-free additive, is known to act as a growth-enhancer and appears to act as an antioxidant compound. However, high dosages may cause detrimental physiological effects to fish. Considering such, a feeding trial was carried out to evaluate the effects of dietary l-carnitine content on neuronal activity (acetylcholinesterasic activity), hepatic metabolism (catalase activity) and liver histological markers in both diploid and triploid trout (Oncorhynchus mykiss) juveniles. Fish were hand-fed for 56 days on three nutritionally identical diets, varying in l-carnitine content; unsupplemented diet (15 mg kg−1) and supplemented (200 or 530 mg kg−1) diets. Moderate to severe glycogen/lipid depletion was the major liver histopathological disorder observed in all dietary groups. This cause–effect relationship appeared to be dose and time-dependent, suggesting an important role of l-carnitine in the liver metabolism. Moreover, the hepatocite size and nuclear diameter appeared to be larger in l-carnitine supplemented groups than in control group. l-carnitine caused significant induction of catalase activity and inhibition of AChE. Diploid and triploid trouts showed marked differences in enzymatic activities, reflected by consistently higher values of AChE activities for triploid animals.