Stress is shown to induce at first the generalized inhibition of lipid peroxidation (LPO), and then the activation of LPO. In brain and blood serum of rats subjected to continuous footshock as well as to restraint stress LPO products decreased and superoxide scavenging activity increased during the initial period of stress, after 1 hour of footshock LPO indices nearly reached normal values, and after 2 hours of footshock the accumulation of LPO products and decrease of superoxide scavenging activity were seen. LPO inhibition was accompanied by accumulation of easy oxidizable brain phospholipids and by depletion of brain cholesterol, during LPO activation brain cholesterol content and cholesterol-phospholipid ratio increased. The content of LPO products--fluorescent Schiff bases in blood plasma of women suffering from algomenorrhea at first decreased (O-12 h) and then dramatically increased (12-24 h) after a onset of pain at the beginning of menstruation. The data suggest that the stage of LPO inhibition precedes its activation during stress.