Exercise training can induce important haemodynamic and metabolic adaptations in patients with chronic heart failure due to severe left ventricular dysfunction. This study examined the impact of exercise rehabilitation on cardiac neuronal function using iodine-123 metaiodobenzylguanidine (MIBG) scintigraphy, Fourteen patients (11 men, 3 women; mean age 48 years; range: 36-66 years) with stable chronic heart failure of NYHA class II-III and an initial resting radionuclide left ventricular ejection fraction (LVEF) < 50% were enrolled in the study. Patients underwent progressive, supervised endurance training (treadmill test, Bruce protocol) during a 6-month period (60 sessions, 3 sessions per week) at a cardiac rehabilitation referral centre in order to measure exercise parameters. Planar 123I-MIBG scintigraphy provided measurements of cardiac neuronal uptake (heart-mediastinum ratio activity, 4 h after intravenous injection of 185 MBq of MIBG). Radionuclide LVEF was also assessed at the outset and after 6 months of exercise training. Workload (801 +/- 428 vs 1229 +/- 245 kpm.min-1, P = 0.001), exercise duration (504 +/- 190 vs 649 +/- 125 s, P = 0.02), and myocardial MIBG uptake (135% +/- 19% vs 156% +/- 25%, P = 0.02) increased significantly after rehabilitation. However, LVEF did not change significantly (23% +/- 9% vs 21% +/- 10%, p = NS). It is concluded that exercise rehabilitation induces improvement of cardiac neuronal function without having negative effects on cardiac contractility in patients with stable chronic heart failure.