Abstract 1. 1. Changes in morphology i.e. “metamorphosis”, mode of respiration, allometric growth of organs, mode of swimming and efficiency of biochemical pathways are briefly reviewed. It is suggested that these processes form the basis for progressive changes in nutrient requirements involving formation and development of several organs, systems and function. 2. 2. Digestive tract morphology changes during ontogenesis and aspects of fish metabolism, protein synthesis rate and body growth rate are interconnected and an attempt is made to explain these processes so as to understand the specificity of larval and juvenile fish nutrient requirements as compared to subadults. 3. 3. Protein and amino acid requirements given the body mass perspective and the generalization of the protein maintenance requirement in protein requirements for maximum growth was estimated to amount to 5–20%. Several cases of amino acid deficiency symptoms showed strong dependence on fish weight (age), but even most numerous studies on salmonids are lacking complete research throughout the life history of one species in defined nutritional and environmental conditions. 4. 4. Larval and juvenile fish have reduced capacity of catabolic adaptability and this fact links them to strictly carnivorous mammals. An attempt is made, for the first time, to relate amino acid needs of fish to young and/or carnivorous mammals. 5. 5. Vitamin requirements of fish are reviewed, taking into account the relationship between body size and time of the first appearance of deficiency symptoms. There are virtuallly no studies on vitamin requirements in larval warm-water fishes and very few on first feeding salmonids. The same applies to the vitamin need in reproductory fish. 6. 6. Fatty acid deficiencies manifest themselves faster in juvenile fish, but larval fish might require separate classes of lipids, phosphatidylcholine, in the diet to develop and grow at all. It seems that the studies on nutrient requirements have so far not used an ontogenetical perspective, but evidence given throughout this work argues that it would be worthwhile.