The influence of gonadal maturation on seawater adaptability at the metabolic level was assessed in the euryhaline teleost Sparus auratus by treatment of immature fish with 17-estradiol (E2) followed by acclimation to different environmental salinities. Fish were injected with coconut oil alone (sham) or containing E2 (10g g–1 body weight) and maintained in seawater (40). After 5days, fish from oil- and E2-implanted groups were sampled. At the same time, fish of both groups were transferred to brackish water (5, hypoosmotic test), seawater (40, transfer test), and hypersaline water (55, hyperosmotic test); 4days after transfer (9days post-implant) all groups were sampled. Data obtained from liver suggest that E2 treatment produced effects comparable to those already reported in literature, including decreased glycogen levels, increased glycolytic potential, and decreased gluconeogenic potential. This, together with the fact that many changes displayed in shams among salinities disappeared in E2-treated fish allow us to suggest that the response of liver carbohydrate metabolism to osmotic acclimation is exceeded by the response elicited by E2 treatment. In gills, E2 treatment produced increased lactate levels, decreased capacity for use of exogenous glucose, and decreased the potential of the pentose phosphate pathway. These findings suggest that the energy demand occurring in gills during osmotic acclimation should be increasingly fuelled by substrates other than exogenous glucose. Finally, data obtained in brain of E2-treated fish suggest a lower necessity of exogenous glucose, increased lactate levels, and decreased glycolytic potential.