Changes in the number of Na-K-ATPase -subunits, Na-K-ATPase activity and glycogen content of the crucian carp (Carassius carassius) brain were examined to elucidate relative roles of energy demand and supply in adaptation to seasonal anoxia. Fish were collected monthly around the year from the wild for immediate laboratory assays. Equilibrium dissociation constant and Hill coefficient of [³H]ouabain binding to brain homogenates were 12.87+-2.86 nM and -1.18+-0.07 in June and 11.93+-2.81 nM and -1.17+-0.06 in February (p>0.05), respectively, suggesting little changes in Na-K-ATPase -subunit composition of the brain between summer and winter. The number of [³H]ouabain binding sites and Na-K-ATPase activity varied seasonally (P<0.001), but did not show clear connection to seasonal changes in oxygen content of the fish habitat. Six weeks exposure of fish to anoxia in the laboratory did not affect Na-K-ATPase activity (p>0.05) confirming the anoxia-resistance of the carp brain Na-pump. Although anoxia did not suppress the Na-pump, direct Q₁₀ effect on Na-K-ATPase at low temperatures resulted in ten times lower catalytic activity in winter than summer. Brain glycogen content showed clear seasonal cycling with the peak value of 203.7+-16.1 micromol/g in February and a 15 times lower minimum (12.9+-1.2) in July. In winter glycogen stores are 15 times larger and ATP requirements of Na-K-ATPase at least ten times less than in summer. Accordingly, brain glycogen stores are sufficient to fuel brain function for about 8 minutes in summer and 16 hours in winter, meaning 150-fold extension of brain anoxia tolerance by seasonal changes in energy supply-demand ratio.