The effect of several agents active on autonomic nervous system functions was tested on brain oxygen autoregulation parameters. It was found that atropine, propranolol and isoproterenol had no influence in abolishing the measured parameters. Phenoxybenzamine, tolazoline and dibenamine all suppress autoregulation. In an additional experimental series, a phenoxybenzamine infusion was given during O2 breathing. The infusion induced, in most cases, an additional rise in TpO2 (tissue pressure of oxygen, which refers to the partial pressure [in mm Hg] of this gas at the measuring tip of the electrode). It is concluded that an a-adrenergic mechanism is part of the autoregulation process. Also, the increase in brain TpO2 induced by 95% O2-S% CO2 breathing seems to be blocked or reversed by a-adrenolytic drugs, thus supporting the thinking that the effect of CO, on cerebral blood flow is at least in part mediated through an a-adrenergic response.