Diltiazem (0.3 mg/kg body weight intravenous in 2 minutes) was administered to 40 patients (24 males, 16 females, mean age 51.55 years) with paroxysmal supraventricular tachyarrhythmias: 7 patients with atrial fibrillation, 6 patients with atrial flutter, 25 patients with paroxysmal supraventricular tachycardia, 2 patients with uncommon atrioventricular reciprocating tachycardia. In patients with atrial fibrillation intravenous diltiazem produced a significant decrease of ventricular response (from 160 +/- 11 to 113.57 +/- 10.34--p less than 0.01). In patients with atrial flutter intravenous diltiazem produced variable effects: an increase in atrio-ventricular block (from 2:1 to 3:1 atrio-ventricular conduction (2 patients); conversion to sinus rhythm (1 patient); change to atrial fibrillation (1 patient); no appreciable change of the basic rhythm (2 patients). In paroxysmal supraventricular tachycardia patients conversion to sinus rhythm occurred in 20/22 patients (91%) treated with intravenous diltiazem (mean conversion time 4.69 minutes). In the 2 patients with uncommon atrioventricular nodal reciprocating tachycardia diltiazem increased P'-R and R-P' intervals without appreciable change of the basic rhythm. No serious side effects from drug administration were noted. Intravenous diltiazem appears to be as a highly effective medication in conversion or control of paroxysmal supraventricular tachyarrhythmias.