The central hemodynamic effects of the peptide endothelin-1 (ET-1) have been investigated in the conscious, normotensive rat. Intracisternal administration of ET-1 (0.01-0.03 nmol) gave rise to an increase in mean arterial pressure with minimal effects on heart rate and was accompanied in some cases by barrel rolling activity. Intracisternal administration of 0.03 nmol ET-1 gave rise to a significant elevation in plasma noradrenaline and adrenaline levels. This elevation in plasma catecholamines was present only in those animals that also exhibited marked behavioral changes. Autoradiographic measurement of cerebral blood flow carried out during the maximum response to 0.03 nmol of intracisternal ET-1 revealed a widespread and profound ischemia throughout the caudal brainstem. Cerebral ischemia is known to activate compensatory circulatory reflexes in the medulla oblongata that result in increased sympathetic and vagal outflow. This is the most likely cause of intracisternal ET-1-induced hypertension. ET-1 is unique in its ability to override the brain's autoregulatory mechanisms and induce ischemia of pathological magnitude.