Abstract 1. 1. Investigation has been made of how the LD 50 and LD 100 of PQ influence the peroxide metabolism enzymes and lipid peroxidation in organ homogenates and blood samples of the rat. 2. 2. The peroxide metabolism enzyme activities were changed in a concentration dependent manner. In general, the effect of the lower doses of PQ on the superoxide dismutase, catalase activities and lipid peroxidation are increased. The glutathione peroxidase activity was decreased. 3. 3. Higher doses of PQ enhanced all of enzyme activities and lipid peroxidation studied.