The objective of this experiment was to test the effect of different dietary lipids in brown trout feeding on the activity of metabolic enzymes, such as hexokinase (HK), glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD). Four iso-nitrogenous and iso-lipidic casein-gelatin based experimental diets with varying dietary sources were formulated. The sources of dietary lipid were menhaden oil (MO diet, rich in highly unsaturated fatty acids), hazelnut oil (HO diet, predominantly oleic acid), a blend of linseed oil and soybean oil (1: 1) (LO+SO diet, linoleic and linolenic acids), and soybean lecithin (LE diet, phospholipids; mostly linoleic acid). Fish with an initial individual weight of similar to 1g were distributed randomly in three replicate 50-L glass aquaria with a total of 30 fish per tank. Each tank received one of the four diets referenced above for 6 weeks. In all cases, the activity of all metabolic enzymes significantly increased (P< 0.05) in comparison to their activities in initial fish. All enzyme activities had their highest values in fish fed LO+SO and lowest in fish fed HO diet. It is very important to note that total polyunsaturated fatty acid (PUFA) content of the lipids is in the same order indicating that activity of these enzymes are elevated with increasing PUFA concentrations.