We tested the hypothesis that early plasma atrial natriuretic factor (ANF) values are related to subsequent functional capacity in patients with acute myocardial infarction (MI). Blood for ANF determination was sampled from 90 male patients (age 66.5 +/- 9.5 (mean +/- SD) years) day 3 post MI. Exercise testing on an upright bicycle ergometer to symptomatic end-points was performed 1 and 6 months after MI in 83 and 78 patients, respectively. A weak, but significant inverse relationship between day 3 plasma ANF levels and exercise duration after MI (1 month: r = -0.27, P = 0.012; 6 months: r = -0.36, P = 0.001) was observed. In the subgroup of patients without effort-associated ischaemia, the relationship was closer (1 month; n = 38, r = -0.57, P < 0.001; t months: n = 33, r = -0.65, P < 0.001). In multivariate analysis, with ANF, patient age and peak creatine kinase MB values as covariates, the relationship remained significant. These findings suggest that in male patients subacute plasma ANF measurements are predictive of exercise capacity following acute MI. The relationship appears to be especially prominent in patients without effort-related ischaemia during exercise.