We have studied the cardiovascular effects of inhaled adenosine in healthy subjects. Adenosine at a concentration of 5 mg.ml-1 is absorbed and produces an increase in heart rate of 6-7 beats per min, blood pressure and skin temperature being unaltered. The rise in heart rate is less than that seen with an infusion of a comparable dose and is unlikely to be adequate for therapeutic termination of supraventricular tachycardia. Higher doses are limited by the poor solubility of adenosine. Inhalation as a method of delivering adenosine has possibilities as an alternative route of administration, but some practical problems remain to be overcome.