In acute cerebral ischemia there are severe damages of endothelium recognized as the stimuli for secretion of endothelin-1, that is a endothelium-derived peptide and seems to be the most potent vasoconstrictor known. The goal of this study is to measure plasma endothelin-1 level in patients with cerebral infarction and determine the relationship between endothelin-1 and ischemic stroke. Plasma level of endothelin-1 was measured in 21 consecutive patients. The measurement was performed 3 times at different stages of stroke. There was a marked increase in plasma endothelin-1 level in the patients and the elevation lasted the entire acute and subacute stage of stroke. There was a correlation between the peptide concentration and infarct volume (r = 0.665, P < 0.01). The result suggests that endothelin-1 plays an important role in the regulation of brain circulation. Apparent and lasting increase in plasma level of endothelin-1 is associated with cerebral ischemia and infarction. The peptide seems to be involved in the pathophysiology of acute cerebral ischemia and have a deleterious effect in the evolution of cerebral infarction.