Multicenter, prospective and randomized study in phase IV. A total of 302 patients will be included in the study within 40-50 institutions in Spain. Patients with regular and monomorphic tachycardia ≥ 120 bpm, QRS ≥ 120 ms and haemodynamically well tolerated will be randomized to receive iv amiodarone (single dosage: 5 mg/kg in 20 minutes) vs iv procainamide (single dosage: 10 mg/kg in 20 minutes). The study period will be 40 minutes from infusion initiation. Study variables include the presence of clinically cardiac significant adverse events: hypotension, syncope, heart failure, symptomatic sinus bradycardia, AV block, proarrhythmia and tachycardia acceleration (<20 lpm)and death. Total observation period will be 24 hours after the end of the infusion.