The effects of adenosine given during sinus node reentrant tachycardia were investigated in two patients. Both patients received intravenous bolus doses of 0.1 mg/kg and 0.15 mg/kg. Adenosine slowed the atrial rate in both patients and terminated the tachycardia in one. These effects of adenosine on sinus node reentrant tachycardia suggest that sinus nodal tissue (as opposed to perinodal atrial tissue) is involved in the tachycardia circuit and that these effects should be considered when adenosine is used as a diagnostic agent in narrow-complex tachycardia.