Since oxygen free radicals may have a role in the pathophysiology of endotoxin shock, we have studied the effects of a wide range of compounds (alpha-tocopherol, reduced glutathione, allopurinol, superoxide dismutase (alone or in combination with catalase) and phenyl butylnitrone) that can act either to remove free radicals as they are generated or to prevent their generation. The effects of these substances on the metabolic and cardiovascular responses to endotoxin were examined in conscious rats. The intravenous infusion of endotoxin (10 mg/kg i.v. given over 4 h) resulted in systemic hypotension, transient tachycardia, an increase in plasma lactate, and an initial hyperglycemia followed, in those rats that died before 24 h, by hypoglycemia. The hypotension and tachycardia produced by endotoxin were not significantly modified by alpha-tocopherol, allopurinol, or superoxide dismutase, alone or in combination with catalase. The tachycardia was attenuated by reduced glutathione and phenyl butylnitrone. alpha-Tocopherol attenuated the initial hyperglycemia produced by endotoxin whilst alpha-tocopherol, allopurinol, and phenyl butylnitrone all significantly attenuated the endotoxin-induced increase in plasma lactate. Among the free radical scavenging systems studied, only alpha-tocopherol and phenyl butylnitrone improved survival. These results suggest a contribution from oxygen-free radicals to the pathophysiology of endotoxemia.