The lipid and fatty acid spectrum of juvenile atlantic salmon (aged 0+, 1+, 2+, 3+ (smolts)) from the Varzuga River (Porokushka rapid) was studied in the summer period. a high content of polyunsaturated fatty acids with the dominance of docosahexaenoic 22:6(n-3) and eicosapentaenoic 20:5(n-3) fatty acids was revealed. Smolts (3+) had an elevated proportion of physiologically active 22:6(n-3), 20:5(n-3) and 20:4(n-6) fatty acids, which is characteristic of marine fish and is associated with their preparation for smoltification and transition to life in the sea. an increased level of monounsaturated fatty acids with the dominance of 18:1(n-9), 16:1(n-7) and 18:1(n-7) acids was shown for juveniles aged 0+, 1+ and 2+. In smolts, however, the level of these fatty acids was reduced, which positively correlates with changes in TaG storage and is characteristic of salmon smolts. as compared to 1+ and 2+ parr, fingerlings (0+) and smolts (3+) had a low content of storage TaGs and the TaG/PL index, which is most likely due to a high metabolic rate. The early development and completion of the preparation of salmon smolts for transition to the marine environment were facilitated by ‘launching’ the mechanisms of biochemical adaptations, including those involving individual lipids and fatty acids that play an important role in maintaining homeostasis in an organism exposed to changing environmental conditions.