We have investigated the temperature effect on the catalytic activity of three enzymes from Antarctic fish: glucose-6-phosphate dehydrogenase (G6PD) from the blood of the white-blooded channichthyid Chionodraco hamatus and the red-blooded nototheniid Dissostichus mawsoni, L-glutamate dehydrogenase (GDH) from the liver of the channichthyid Chaenocephalus aceratus and glycogen phosphorylase b from the muscle of the nototheniid Trematomus bernacchii. The study of the temperature effect on the catalytic activity of G6PD and GDH shows that the Antarctic enzymes are more efficient catalysts at low temperature than the mesophilic homologous enzymes. The activity of Antarctic G6PD increases at increasing temperatures; above 55°C irreversible heat inactivation occurs. A different behavior is shown by C. aceratus GDH. This enzyme has an apparent optimal temperature at 25°C; above this temperature, the catalytic activity decreases and irreversible heat inactivation is then observed above 45°C. All three enzymes are irreversibly inactivated at temperatures much higher than the physiological ones and only slightly lower than those of the homologous mesophilic enzymes. In Antarctic phosphorylase b, the allosteric activator AMP protects against inactivation. In general, heat inactivation of these enzymes occurs very abruptly in a narrow temperature range.