Catalase activity was evaluated in Long Evans rat retina after ischemia and reperfusion. Ischemia was induced by ligation of the optic nerve and vessels. Rats were sacrificed after 15 and 120 min of reperfusion, respectively. Catalase activity was assessed by Claiborne's method and was expressed as U/mg of protein. In the first group, retinas of each animal were pooled. In the second group, ischemia was induced in the right eye with the left eye serving as control. In the first group, enzyme activity was 7.39 +/- 0.26 (n = 11), 7.67 +/- 0.27 (n = 9) and 9.15 +/- 0.45 (n = 7) for the sham-operated, 15- and 120-min reperfusion groups, respectively. There was a significant difference between the control and 120-min reperfusion groups (p < 0.001). In the second group, there was a significant (p < 0.01) increase in catalase activity in the ischemic eye compared to the non-ischemic eye after 15 (n = 7) and 120 min (n = 9) of reperfusion. These findings may suggest a rapid activation of catalase activity during the ischemia-reperfusion sequence.