Ten normal subjects and 14 patients with chronic Chagas' disease (seven with and seven without heart disease) underwent dynamic exercise on a cycle ergometer. Heart rate (HR), pulmonary ventilation (V), oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory quotient (RQ) were measured. Increasing workloads (25, 50, 100, and 150 W) were applied for 4 min and intercalated with resting periods. The main objective of this protocol was to analyse heart rate response in relation to the other cardiorespiratory variables in order to evaluate the functional conditions of the sympathetic and parasympathetic cardiac efferents. Analysis of the results showed that (a) the group of chagasic patients with heart disease had lower heart rates (p less than 0.05) than normal subjects during the initial 10 s (delta HR 0-10 s) of effort (fast component); (b) the difference between the normal subjects and chagasic patients without heart disease was not statistically significant; (c) the abnormalities in heart rate response were due to depression of parasympathetic efferent action on the sinus node; (d) the slow heart rate response (delta HR 1-4 min), which expresses the degree of sympathetic stimulation of the sinus node, was comparable in the three groups studied, thus showing unimpaired adrenergic responses during dynamic exercise in Chagas' disease; and (e) the V, VO2, VCO2, and RQ values were normal at all workloads in each group, suggesting that vagal dysfunction does not affect oxygen transport at these submaximal levels of dynamic exercise.