To characterize the change in cycle length and QRS morphology before spontaneous termination of sustained ventricular tachycardia (VT), electrocardiograms were recorded and VT cycle length measured for the periods 31 to 21 and 11 to 1 beats before termination in 55 episodes from 28 patients with coronary artery disease. Beats 31 to 21 were designated as a period of stable arrhythmia and served as a reference for changes occurring just before termination. Forty-four episodes of VT occurred in the setting of antiarrhythmic drug therapy; 11 episodes occurred in patients not treated with antiarrhythmic drugs. Variability in cycle length was indexed by the standard deviation of the mean cycle length and by the percentage of consecutive cycles varying by greater than or equal to 40 ms (% greater than or equal to 40 ms). There was greater variability just before termination (standard deviation of the mean cycle length, 25.8 ms; % greater than or equal to 40 ms, 16.7%) than during the stable period (standard deviation of the mean cycle length, 8.5 ms; % greater than or equal to 40 ms, 5.4%; p less than 0.001 for both). This was true irrespective of antiarrhythmic drug use, although the differences in the standard deviation of the mean cycle length for beats 11 to 1 and for beats 31 to 21 were greater for the antiarrhythmic drug group (29.6 vs 8.9 ms, p less than 0.001) than for the group not receiving antiarrhythmic drugs (11.0 vs 6.7 ms, difference not significant). No specific patterns of cycle length variability characteristic of VT termination were found.(ABSTRACT TRUNCATED AT 250 WORDS)