We tested the hypothesis that along with gill Na+K+ATPase activity reduction, branchial energetic metabolism of brook charr (Salvelinus fontinalis) is altered during sexual maturation. Maturing and sterile fish were transferred from freshwater (FW) to seawater (SW). The activity of gill pyruvate kinase (PK), cytochrome C oxidase (COX), citrate synthase (CS) and lactate dehydrogenase (LDH) was measured at different intervals. Following 1 month in SW, twofold increases in Na+K+ATPase activity were recorded in all groups, while mean metabolic enzyme activities were sharply reduced (COX: 0.051 ± 0.029 to 0.036 ± 0.018; PK: 1.670 ± 0565 to 1.210 ± 0.340 LDH: 2.245 ± 0.690 to 1.642 ± 0.381 μmol mg–1 protein min–1). Interestingly, during this period, no mortality occurred. After 5 months, comparison of Na+K+ATPase:CS and Na+K+ATPase:LDH ratios of mature and sterile fish held either in FW or SW, indicated that the response of Na+K+ATPase largely exceeds the response of the metabolic enzyme apparatus in hyperosmotic conditions. Hence, the reduced iono-osmoregulatory capacity and higher mortality observed in SW-maturing fish during the reproductive season appears to be mainly attributable to Na+K+ATPase activity reduction rather than the alteration of gill metabolic capacity, since no concurrent increase of metabolic enzyme activity with Na+K+ATPase activity occurred.