Elasmobranch fishes have long been noted for having unusually large brains for ectotherms, and therefore may be exceptions to the rule that vertebrates in general devote less than 8% of their resting metabolic rate to the central nervous system. The brain mass of sharks, skates and rays is often several times larger than that of teleost fishes of the same size. Still, the underlying reasons for this have remained unclear. Ion pumping by the Na+/K+–ATPase is the single most energy consuming process in the brain. In this study, Na+/K+–ATPase activity was measured in the brain of four species of elasmobranchs and 11 species of teleosts. While the average brain mass of the elasmobranchs examined was approximately three times that of the teleosts, the mean specific Na+/K+–ATPase activity was only about one–third of that of the teleosts. Thus, the total brain Na+/K+–ATPase activity was similar in elasmobranchs and teleosts. This suggests that the large brain size of elasmobranchs is at least partly related to a low mass–specific rate of brain energy use.