UNLABELLED: The safety and therapeutic efficacy (sinus rythm recovery) of intravenous diltiazem vs verapamil in paroxysmal supraventricular tachycardias (PSVT), and diltiazem vs the Sokolow protocol (i.v. digital and oral quinidine) in recent onset atrial fibrillation (AF) were compared. Sixty patients with PSVT were randomized to have a bolus of 0.3 mg/Kg of diltiazem or 75 micrograms/Kg of verapamil. If after 15 minutes the PSVT persisted, a 6 hours i.v. infusion of diltiazem was started (0.0028 mg/Kg/min) or a second dose of verapamil was repeated. In 100 patients with PAF the alternative to diltiazem was i.v. digital plus oral quinidine (400 to 1,200 mg). Ninety per cent of the PSVT and 64% of de AF, recovered sinus rythm with diltiazem. The same results were obtained with verapamil in PSVT. With the Sokolow protocol 90% of the AF recovered sinus rythm. In 83% of AF who did not revert to sinus rythm, diltiazem was effective for slowing the ventricular response. In relation with the drug safety, only 5 patients showed hypotension, without clinical relevance, in the diltiazem group. With verapamil one patient had a transitory ischemic attack after recover sinus rythm. In the Sokolow group 26% had minor gastrointestinal disorders. CONCLUSIONS: In this clinical trial (n = 160): 1) Diltiazem and verapamil were highly and equally effective (90%) in reverting PSVT to sinus rythm. 2) The Sokolow protocol was more effective than diltiazem for reversing PAF to sinus rythm (90% vs 64%, p < 0.01). 3) Diltiazem was effective for slowing the ventricular rate in 83% of the patients with AF who did not revert to sinus rythm. 4) There was a low incidence of side effects with diltiazem and verapamil. 5) Diltiazem is a first choice therapeutic agent in reverting PSVT to sinus rythm and for slowing the ventricular rate in AF.