Adenosine was administered in increasing intravenous bolus doses to nine healthy volunteers. A transient decrease in heart rate was observed which occurred about 20 s after injection. This was due to a reduction in sinus rate and/or transient block of conduction through the atrioventricular node, and lasted less than 10 s. An increase in sinus rate which followed any transient bradycardia was also observed. The maximum bradycardia and maximum tachycardia were each significantly related to the logarithm of adenosine dose.