Adenosine, a naturally occurring nucleoside that slows conduction through the atrioventricular node, has recently been approved for the treatment of supraventricular tachycardia. It has been shown to convert patients with supraventricular tachycardia to sinus rhythm in up to 92% of cases. Its intravascular half-life of only 10 seconds and absence of reported serious side effects have made adenosine an attractive antiarrhythmic agent. This report describes two cases in which significant side effects from the administration of adenosine were encountered including: (1) prolonged sinus arrest with syncope, and (2) syncope with prolonged bradycardia and hypotension. Emergency physicians should be cognizant of the potential complications resulting from adenosine administration, and should be prepared to deal with them when using this newly available agent.