Plasma levels of neuropeptide Y-like immunoreactivity (NPY-LI) and noradrenaline were studied for 25 h in 22 patients with acute ischaemic heart disease. On admission, NPY-LI levels were above normal in 16 patients, and 20 patients had increased noradrenaline levels. The initial plasma NPY-LI did not differ between patients with acute myocardial infarction (AMI) and angina pectoris. Initial plasma noradrenaline levels were higher in patients with AMI than in those with angina pectoris. Plasma levels of noradrenaline remained elevated in AMI patients, but decreased towards normal values in patients with angina pectoris. Levels of NPY-LI returned to normal within 25 h in all patients. Tachycardia and left ventricular failure were related to high NPY-LI and noradrenaline levels. A positive correlation was found between noradrenaline and NPY-LI in plasma. It is suggested that neuropeptide Y (NPY), an endogenous vasoconstrictor peptide, should be considered as one of the mediators involved in the cardiovascular response to sympathetic activation induced by myocardial ischaemia.