Measurements of plasma pancreatic polypeptide and gastrin are reported for the first time in patients with acute myocardial infarction and compared with clinical signs of vagal or sympathetic overactivity. Pancreatic polypeptide concentrations were assessed as an index of vagal activity, but elevated values of pancreatic polypeptide found in 7 of the 13 patients on admission did not correlate with clinical evidence of vagal overactivity. The mean pancreatic polypeptide concentrations were not higher in patients with clinical vagal overactivity than in patients with clinical sympathetic overactivity during the 12 h after the onset of symptoms of acute myocardial infarction. Mean gastrin levels were significantly higher on admission and at 4, 5, 6 and 8 h after the onset of infarction in the patients with clinical features of sympathetic overactivity than in the patients with clinical vagal overactivity. Thus plasma gastrin warrants further assessment as an index of sympathetic overactivity in acute myocardial infarction.