Neuropeptide Y (NPY) is a peptide released together with noradrenaline (NA) from sympathetic nerve endings. Elevated plasma levels of NA and NPY-like immunoreactivity (LI) are found in patients with congestive heart failure. In order to assess any relationship found between plasma NPY-LI and haemodynamic data 12 patients with mild to moderate chronic congestive heart failure were studied during cardiac catheterization. All patients were treated with diuretics but not ACE inhibitors and were in New York Heart Association functional class II or III. Mean left ventricular ejection fraction determined by echocardiography was 34%. Plasma NPY-LI from mixed venous blood was elevated in five patients and NA in nine. Mean plasma NPY-LI was 29 +/- 3 pmoll-1 (mean +/- standard error of the mean) and NA 2.6 +/- 0.3 nmoll-1. Heart rate was 70 +/- 3 beats min-1, systolic blood pressure (SBP) 131 +/- 7 mmHg, stroke volume index (SVI) 29 +/- 1.9 ml m-2 and cardiac index (CI) 2.0 +/- 0.13 l min-1 m-2. Elevated levels of plasma NPY-LI (> 30 pmol l-1) were associated with lower SVI, CI, and SBP and a higher pulmonary vascular resistance. Elevated plasma NA (> 2 nmol l-1) did not correlate with haemodynamic data. Log NPY-LI correlated inversely with SVI (P < 0.01) and CI (P < 0.05) but not with plasma NA. It is concluded that log NPY-LI in mixed venous blood correlates inversely with SVI and CI in patients with mild to moderate chronic congestive heart failure.