We evaluated alterations in cardiac adrenergic neuron activity and progression of left ventricular dysfunction in comparison with the severity of structural changes using a rat model of adriamycin cardiomyopathy. Rats were treated with adriamycin (2 mg/kg s.c. once a week) for 6, 7, 8 and 9 wk. Accumulation of 125I-metaiodobenzylguanidine (MIBG) 4 hr after intravenous administration was determined and left ventricular ejection fraction (LVEF) was calculated from gated blood-pool images. H & E and Masson-Trichrome stained specimens of the myocardium were examined by light microscopy. Histopathologic examination demonstrated dose-dependent myocyte damage, although there were no differences between the 8-wk and 9-wk groups. LVEF did not differ between controls and the 6-wk group (81.3% +/- 5.5% versus 82.1% +/- 4.8%, p = ns). LVEF began to decrease slightly in the 7-wk group (75.0% +/- 5.7%, p < 0.05) and showed a remarkable decrease in the 8-wk group (53.7% +/- 2.6%, p < 0.001). In the 9-wk group, LVEF diminished to 47.9% +/- 3.1% (p < 0.001), accompanied by massive pleural effusions and ascites. MIBG accumulation in the heart (%ID/heart) significantly and progressively diminished; 1.42% +/- 0.15% in the 6-wk group, 1.06% +/- 0.16% in the 7-wk group, 0.77% +/- 0.13% in the 8-wk group and 0.34% +/- 0.11% in the 9-wk group, respectively p < 0.001, compared to controls (1.99% +/- 0.30%). These results demonstrate that MIBG accumulation in the heart showed a greater and more linear dose-dependent decrease than LVEF. Furthermore, MIBG uptake was significantly reduced in the 6-wk group where only mild myocyte damage (isolated vacuolation or myofibrillar loss) was observed. Thus, MIBG may be a sensitive biochemical marker of adriamycin cardiomyopathy.