The effects of intravenous administration of adrenoceptor agonists and antagonists on relative cerebral blood flow, cerebral perfusion pressure, intracranial pressure, mean arterial blood pressure and heart rate were assessed in rats under urethane anesthesia. Administration of phenylephrine (a preferential alpha 1-adrenoceptor agonist), adrenaline (a mixed alpha/beta-adrenoceptor agonist), noradrenaline (a mixed alpha/beta-adrenoceptor agonist) raised mean arterial pressure, cerebral perfusion pressure, cerebral blood flow and intracranial pressure, but lowered heart rate. On the other hand, administration of isoproterenol (a beta-adrenoceptor agonist), phentolamine (an alpha-adrenoceptor antagonist) or propranolol (a beta-adrenoceptor antagonist) lowered mean arterial pressure, cerebral perfusion pressure or cerebral blood flow. In addition, phentolamine raised both intracranial pressure and heart rate, whereas propranolol lowered both intracranial pressure and heart rate. However, isoproterenol produced a decrease in heart rate, without affecting intracranial pressure. There was no significant difference between the groups of animals for PCO2, PO2 or pH throughout the studies. The results suggest that adrenoceptor agonists or antagonists act through breakthrough of autoregulation, with acute hypertension or hypotension, to enhance or to reduce cerebral blood flow in rats.