The effects of clonidine were studied on insulin secretion from the isolated pancreas of the rat and newborn dog perfused in presence of 8.3 mmol/l glucose. The effects were compared to those induced by adrenalin. In the rat, clonidine and adrenalin infused for 30 minutes at 0.01 mumol/l caused an immediate and marked inhibition of insulin release (-- 79.1 +/- 2.6% for clonidine and -- 75.7 +/- 4.7% for adrenaline at the 15th minute of infusion). The inhibition due to clonidine has not completely disappeared 15 minutes after stopping the infusion. In the dog, both substances also induced an immediate and marked inhibition; but the effect of adrenalin disappeared before the end of the infusion. It follows that the inhibitory effect of clonidine on insulin secretion is more durable than that of adrenalin.