The crucian carp is an exceptionally anoxia-tolerant vertebrate. For the brain, with its very high rate of ATP use, depression of energy use is likely to be an important strategy for anoxic survival. This study shows that the light-evoked response of the retina and the corresponding evoked potential in optic tectum decrease in amplitude by 69 and 75%, respectively, during 38 min of anoxia, and by about 90% after 1 h in anoxia. Both responses were restored upon reoxygenation. The length of light exposure (5 s or 100 ms) did not affect the degree of anoxic depression. These results are the first to show an anoxia-induced depression of central nervous system (CNS) activity in vivo in this species, and indicate that the crucian carp temporarily turns off its visual sense in order to reduce neural energy use during anoxic condition.