Brain natriuretic peptide (BNP) is a cardiac ventricular hormone that may be a sensitive and specific marker of changes in ventricular function. In a prospective, randomised open trial with 16 patients followed for 6 months after first Q wave anterior myocardial infarction we set out to determine: whether BNP concentrations are raised acutely, the effect on circulating BNP of angiotensin-converting enzyme (ACE) inhibition, how BNP and atrial natriuretic peptide (ANP) concentrations compared as correlates of left-ventricular ejection fraction, and whether plasma BNP concentrations could distinguish patients with low (< 40%) and relatively preserved (> 40%) ejection fractions. Plasma concentrations of BNP measured on days 2, 7, 8, 42, and 180 postinfarction were significantly raised in patients compared with normal controls and to a proportionately greater degree than ANP concentrations. Treatment with placebo (n = 8) or oral captopril (n = 8) from day 8 resulted in significantly lower BNP concentrations at days 42 (p = 0.05) and 180 (p < 0.05) in the captopril-treated group. Compared with ANP, BNP concentrations were much more strongly correlated with radionuclide-measured left-ventricular ejection fraction at days 2, 42, and 180. All 8 patients with baseline (day 2) ejection fractions of 40% or above had plasma BNP concentrations less than 10 pmol/L, whereas the 8 patients with ejection fractions less than 40% had BNP concentrations greater than 10 pmol/L. Our findings suggest that measurements of circulating BNP may identify those patients with significant left-ventricular dysfunction who have been highlighted by the Survival and Ventricular Enlargement study as likely to benefit from long-term ACE inhibition after myocardial infarction.