The electrophysiologic effects and efficacy of diltiazem were evaluated with programmed electrical stimulation of the heart in 12 patients with supraventricular re-entrant tachycardia (five with atrioventricular nodal tachycardia and seven with circus movement tachycardia the accessory pathway being concealed in 4). Diltiazem was infused over 1 minute at the dose of 0.25 mg/kg and the electrophysiologic parameters were evaluated at 5 and 30 minutes. Diltiazem prolonged the AH interval from 83.5 +/- 58 to 99 +/- 55 msec (P less than 0.05), effective and functional refractory periods of atrioventricular node from 244 +/- 40 to 268 +/- 56 msec (P less than 0.05) and from 432 +/- 124 to 468 +/- 130 msec (P less than 0.005) respectively, lowered the atrial pacing rate inducing second-degree atrioventricular block from 159 +/- 32 to 134 +/- 33 beats/min (P less than 0.005) and decreased systolic and diastolic blood pressure from 143.5 +/- 33 to 132.5 +/- 22 mm Hg (P less than 0.05) and from 90 +/- 15 to 82.5 +/- 9 (P less than 0.05), respectively. Diltiazem prevented the reinduction of the tachycardia in 4 of 5 patients with atrioventricular nodal tachycardia and in 4 of 7 with circus movement tachycardia. The mechanism of action of diltiazem consisted of depression of conduction in atrioventricular node in anterograde fashion while there were no effects on refractoriness of the accessory pathway. The drug was well tolerated with no adverse effects. Diltiazem, therefore, appears an effective and safe drug in the acute treatment of re-entrant supraventricular tachycardia.