1. Selenium-dependent, selenium-independent and total glutathione peroxidase activities were measured in liver, kidney, heart, lung, intestine and stomach of a variety of animals, mostly mammals. 2. Total glutathione peroxidase activity of liver was highest in the order: hamster greater than gerbil approximately equal to rabbit greater than mouse greater than rat. Total glutathione peroxidase activity was adequate in the livers of other mammals and in the kidney, heart, lung, intestine and stomach of all mammals. 3. The total glutathione peroxidase activities in the following tissues correlated significantly: liver and kidney; heart and lung; and intestine and stomach. Selenium-dependent glutathione peroxidase activities in rat tissues correlated with reported activities of copper-superoxide dismutase in the same tissues. 4. Considering the phylogenetic distribution of total glutathione peroxidase activities, the rodent limb of the phylogenetic tree has the highest activities.