Among 200 consecutive cases of acute myocardial infarction (AMI) treated in a CCU, 117 episodes of slow ventricular tachycardia were observed in 72 patients. This figure represents a 36% rate of incidence. It is a relatively high figure because of the close monitoring to which the patient is submitted and because of the early admission to the unit. There were no significant differences of age, sex, or localization of the myocardial necrosis between patients with SVT and those without it. The different mechanisms of production described support an active origin in most of the patients for the following reasons: (1) coexistence of SVT and PVT in 51.3% of the patients; (2) identical QRS morphology in both rhythms; (3) onset of the SVT after a nonprolonged diastole in 70% of the tracings; (4) inhibition of the SVT after increase of the sinus discharge in only 14 occasions; and (5) irregular SVT rhythm in 76.9% of the recordings and ectopic mechanisms with different degrees of exit block. Because of the potential hazard of the SVT, especially if it is assumed to be of an active origin, we recommend lidocaine for patients with a sinus rate faster than 60 per minute or coexisting PVT. Atropine should be used when the sinus rate is slower than 60 per minute assuming a possible escape or passive origin.