The efficacy of diltiazem was studied with programmed electrical stimulation of the heart in 13 patients with inducible tachycardia (atrioventricular nodal tachycardia: 8 cases; orthodromic circus movement tachycardia involving an accessory pathway: 4 cases; ventricular tachycardia: one case). Diltiazem was administered intravenously at a dose of 0.25 mg kg-1 over 2 min. It lengthened the transnodal conduction time, and the effective and the functional refractory periods of the AV node. The shortest pacing cycle length with 1:1 conduction through the node lengthened both in the anterograde and retrograde directions. Diltiazem did not alter anterograde or retrograde refractoriness of accessory pathways. Infused intravenously during episodes of tachycardia, diltiazem interrupted the arrhythmia in all patients. Tachycardia could still be initiated in 2 patients after drug, but the arrhythmia was not sustained in one case. One patient with preexcitation and atrial fibrillation exhibited an increase in ventricular rate after diltiazem. The shortest RR intervals before and after diltiazem were 240 ms and 180 ms, respectively. Thus, iv diltiazem is an effective antiarrhythmic drug for patients with reciprocal supraventricular tachycardia and for selected patients with ventricular tachycardia.