Vertebrates have repeatedly been noted for having remarkably constant ratios of brain to body O2 consumption, the brain using 2-8 % of resting body O2 consumption, suggesting that evolution has put strict limits on the energetic cost of brain function. Only man, with a value of 20 %, is an exception to this rule. However, the results presented here suggest that, in the electric fish Gnathonemus petersii, the brain is responsible for approximately 60 % of body O2 consumption, a figure three times higher than that for any other vertebrate studied, including man. The exceptionally high energetic cost of the G. petersii brain appears to be a consequence both of the brain being very large and of the fish being ectothermic. It was also found that G. petersii has a high ability to utilise O2 at low levels. Thus, during falling [O2], this species was found to maintain both its O2 uptake and its electric discharge rate down to an ambient O2 level of 0.8 mg l-1 (at 26 °C), although it was unable to tolerate an [O2] below 0.3 mg l-1. During severe hypoxia (<0.8 mg l-1), G. petersii attempted to gulp air from the water surface. These results establish a new record for the energetic cost of a vertebrate brain and they show that the species possessing such a brain has a high capacity for utilising O2 at very low ambient concentrations.