Intravenous diltiazem has been used to rapidly reduce ventricular rate in patients with supraventricular tachyarrhythmias (SVTs). This study assesses the efficacy and safety of a low-dose (0.1 mg/kg) intravenous bolus of diltiazem on the heart rate and blood pressure. Fifteen consecutive patients were treated for 21 episodes of atrial fibrillation, sinus tachycardia or ectopic atrial tachycardia. The bolus dose reduced the heart rate significantly in all patients with atrial fibrillation within 5 min (173 +/- 21 to 144 +/- 27 beats/min, p < 0.05) with the maximum response seen at 10 min (128 +/- 26.7 beats/min, p < 0.05). However, patients with sinus tachycardia showed a significant decrease in heart rate only at 15 min (147 +/- 12 to 123 +/- 15 beats/min, p < 0.05). One patient with ectopic atrial tachycardia required maintenance infusion of diltiazem. Conversion to sinus rhythm was observed in another patient with atrial fibrillation. The drug was well tolerated with no significant hypotension, bradyarrhythmia or aggravation of pre-existing congestive heart failure. It is concluded that Indian patients require a lower dose of intravenous diltiazem as compared to Western ones (0.1 mg/kg versus 0.25-0.33 mg/kg) which gives a prompt, effective and safe response in patients with SVTs.