The sympathetic nervous system is activated in acute myocardial infarction (MI). Scarce data exist, however, regarding the release of the sympathetic cotransmitter neuropeptide Y (NPY) during the acute and early convalescent phases after acute MI. Plasma NPY determination was obtained on days 1 and 3 after admission from 47 patients with acute MI and from eight control patients with acute chest pain without MI. Samples were also obtained on day 30 from the 39 survivors from the original MI cohort. Plasma NPY peaked on day 3 in the MI group (day 1: mean = 46.0 pmol/L, SEM = 6.4 pmol/L; day 3: mean = 60.8 pmol/L, SEM = 5.7 pmol/L; day 30: mean = 27.2 pmol/L, SEM = 4.1 pmol/L; days 1 to 3: p = 0.002; days 3 to 30: p < 0.001), whereas in the control group a nonsignificant decrease from day 1 (mean = 42.6 pmol/L, SEM = 12.3 pmol/L) to day 3 (mean = 34.0 pmol/L, SEM = 5.6 pmol/L) was observed. Plasma NPY levels were significantly increased in patients with MI on day 3 (p = 0.044), but not at baseline compared with the control group. No significant association between plasma NPY and plasma catecholamines, clinical heart failure, or 1-month survival was evident. These results suggest that increased plasma levels of the vasoconstrictory and cardiodepressant sympathetic neurotransmitter NPY are present in the recovery phase of MI, but with a plasma profile distinct from that of catecholamines.