The effects of diltiazem hydrochloride (0.3 mg/kg i.v. over 2 min.) was studied by continuous electrocardiographic monitoring in 60 patients. Conversion to sinus rhythm was achieved in 55 patients (91%). Electrocardiographic findings were: undisturbed sinus rhythm in 20 patients; A-V junctional rhythm in 4 patients; complex ventricular arrhythmias (ventricular tachycardia or complex VPCs) in 19 patients. Patients with complex ventricular arrhythmias were matched against patients with normal sinus rhythm, with respect to the following parameters: age, sex, heart disease, tachycardia duration, tachycardia cycle length, sinus cycle length, pre- and post-infusion blood pressure. No differences between the two groups of patients were found. Ventricular arrhythmias occurring at the termination of supraventricular tachycardia are difficult to explain. Nevertheless, these arrhythmias are not associated with organic heart disease. They could be the expression of triggered activity.