A time course (48 hours) of plasma neuropeptide Y (NPY) levels has been carried out in a male, 66 years old, admitted to Coronary Care Unit with inferior acute myocardial infarction within 1 hour from the onset of chest pain. On admission an increase of plasma NPY levels (38 pg/ml) has been observed. The plasma NPY value decreased to normal range (15-25 pg/ml) within 12 hours and increased again (53 pg/ml) within 12 and 24 hours. A decrease in plasma NPY values to normal range has been observed within the second day in the Coronary Care Unit. A clearcut diuresis decrease, without pulmonary signs of heart failure, was present from 12 to 24 hours followed by marked polyuria within the second day. These data point out a relative importance of NPY in the diuresis adjustments. Thus, plasma NPY measurement might be a more reliable prognostic indicator of heart failure than plasma catecholamine levels. However, further investigations have to be performed.