We studied 28 cases of chronic chagasic myocarditis (CCM) with frequent ventricular arrhythmias. Two-hundred and three conventional ECGs recorded during 3 months showed ventricular extrasystoles (VE) ranging between 0.2 and 6 per ten beats in 100%; multiform VE in 97.04%; couplets in 79.31%; ventricular tachycardia (VT) in 42.85%; and R on T in 21.67%. A 24-hour continuous recording showed that VE ranged between 3780 and 61733 (mean 16618 +/- 2627); multiform VE and couplets were present in 100% of patients, and VT was present in 78.5%. In 16 patients (group I) the frequency of VE was persistently high, without diurnal variation; 11 patients showed sustained reduction during sleeping hours and only one showed an increase during night sleep (group II). Even in group II, VE never disappeared for periods longer than 10 minutes. In five patients, four 24-hour recordings were obtained at weekly intervals, and in five other patients a second 24-hour recording was performed 10 to 24 months later. The remarkable frequency, persistence and low variability of ventricular arrhythmias in CCM suggest that such arrhythmias can be used as a most stable, reliable, but highly demanding model for testing the efficacy of antiarrhythmic drugs.