Catalase (CAT, EC 220.127.116.11) is an important enzyme in antioxidant defense system protecting animals from oxidative stress. Freshwater fish Oreochromis niloticus were exposed for 96 h to different concentrations of Ag+, Cd2+, Cr6+, Cu2+ and Zn2+, known to cause oxidative stress, and subsequently CAT activities in liver, kidney, gill, intestine and brain were measured. In vivo, CAT was stimulated by all metals except Ag+ in the liver and the highest increase in CAT activity (183%) resulted from 1.0 mg Cd2+/L exposure, whereas 0.5 mg Ag+/L exposure resulted in a sharp decrease (44%). In tilapia kidney, cadmium and zinc had no significant effects on CAT activity, whereas 0.1 mg Cr6+/L exposure caused a decrease (44%). Cadmium and zinc did not significantly affect the CAT activity in gill; however, 0.5 mg Ag+/L exposure caused an increase (66%) and 1.5 mg Cr6+/L exposure caused a decrease (97%) in CAT activity. All metals, except Cu2+(41% increase), caused significant decreases in CAT activity in the intestine. In brain, 1.0 mg Zn2+/L resulted in an increase in CAT activity (126%), while 1.5 mg Ag+/L exposure caused a 54% decrease. In vitro, all metals - except Ag+ and Cu2+ in kidney - significantly inhibited the CAT activity in all tissues. Results emphasized that CAT may be considered as a sensitive bioindicator of the antioxidant defense system. (c) 2006 Elsevier Inc. All rights reserved.