The effect of increased stimulus current strength on the right ventricular effective refractory period during sustained ventricular tachycardia and on the ability of single premature right ventricular stimuli to terminate ventricular tachycardia was evaluated. Studies were performed during 53 episodes of sustained ventricular tachycardia in 25 patients. Forty-four of 53 episodes were slowed by pharmacologic therapy. Current intensities of twice diastolic threshold, 5 mA and 10 mA were used. Increasing current from twice diastolic threshold (1.52 +/- 0.66 mA) to 5 mA (32 episodes) shortened the mean ventricular effective refractory period from 213 +/- 50 to 188 +/- 43 msec (p less than 0.001); and from twice diastolic threshold (1.53 +/- 0.60 mA) to 10 mA (42 episodes), from 206 +/- 55 to 176 +/- 50 msec (p less than 0.001). There was a direct correlation between cycle length of ventricular tachycardia and the ventricular effective refractory period measured at twice diastolic threshold (r = 0.77, p less than 0.001). However, the cycle length of ventricular tachycardia and the amount of shortening of ventricular effective refractory period as current was increased were not significantly correlated (r = 0.40). Eleven episodes of ventricular tachycardia (21%) that could not be terminated by ventricular stimulation at twice diastolic threshold were terminated when increased current strength was used. The ability to terminate ventricular tachycardia was associated with a decrease in ventricular effective refractory period and long tachycardia cycle length. The mean cycle length of the 11 episodes terminated was 455 +/- 50 msec, compared with 381 +/- 63 msec in the 42 episodes not terminated at increased current.