The in vivo effect of adenosine on the serum levels of glucose, insulin and glucagon in rats fasted for twenty four hours or after an oral glucose load were studied. Under fasting conditions adenosine produced an hyperglycaemia without change in the insulin or glucagon serum levels. After a glucose load adenosine induced a marked hyperglycaemia concomitant to a decrease in insulin serum levels and an increase in glucagon serum levels. Adenosine did not alter the relationship between insulin and glucagon. In vivo adenosine administration altered the secretion of hormones by the islets of Langerhans (increased the release of glucagon and decreased the secretion of insulin) but this was only clearly observable under stimulated conditions. Adenosine did not alter the regulatory mechanism(s) that modulate the relationship between insulin and glucagon.