Cardiac autonomic impairment and right side heart failure are prominent features in patients with Chagas' disease, but no causal relationship between these phenomena has been disclosed and the pathophysiology of such manifestations is unclear. Aim of study was to assess the cardiac autonomic control and biventricular function in chagasic patients in early stages of the disease, using radionuclide angiography, Valsalva manoeuvre, head-up tilt and baroreflex sensitivity evaluation. Thirty-one chagasic patients with no clinical signs of Chagas' heart disease-16 in the indeterminate phase and 15 with sole organic digestive involvement-were studied, and results compared with those obtained in 14 normal volunteers. No significant differences were observed among the three groups, in regard to any systolic or diastolic parameter of LV function, including ejection fraction, peak ejection and filling rates and correspondent times, time to end-systole, and the standard deviation of phase values. The indeterminate and digestive groups of chagasics had significantly lower right ventricular ejection fraction (45.7 +/- 6.3 and 46.2 +/- 10.1 respectively) and peak ejection rate (respectively 2.8 +/- 0.6 and 2.9 +/- 0.6) and higher right ventricular phase standard deviation (22.4 +/- 5.9 and 20.1 +/- 5.6 degrees, respectively), as compared with the control group (53.6 +/- 4.3, 3.5 +/- 0.5, and 15.8 +/- 3.8 respectively for right ventricular ejection fraction, peak ejection rate and phase standard deviation). No significant differences were found between the results of autonomic evaluation in the control and indeterminate groups of chagasic patients. The group of digestive disease patients showed abnormally lower Valsalva ratio (1.5 +/- 0.15), baroreflex sensitivity (8.85 +/- 2.05 ms/mmHg) and parasympathetically-dependent heart rate response to tilt (8.85 +/- 8.42 beats/mm) and higher Valsalva delay (15.67 +/- 1.35 s) values, compared with the control group (respectively 1.85 +/- 0.49, 20.23 +/- 12.66 ms/mmHg, 21.61 +/- 5.77 beats/mm and 10.1 +/- 2.5 s). Thus, cardiac autonomic impairment is a prominent feature in chagasic patients with the digestive but not the indeterminate form of Chagas' disease. It bears no causative relationship to the early myocardial damage that is apparent only regarding right ventricular function, in both groups of patients. Early right ventricular dysfunction is a likely mechanism for the marked predominance of systemic over pulmonary congestion when heart failure supervenes in patients with Chagas' disease.