In cats, xylazine, an analogue of clonidine, produced hyperglycaemia when injected intravenously. The effect was obtained in unanaesthetized cats and in pentobarbitone sodium anaesthesia. The hyperglycaemia was not a central effect, nor due to adrenaline release from the adrenals, nor to a direct action of xylazine on the liver. It resulted from a fall in plasma insulin produced by an action of xylazine on the pancreas, inhibiting insulin secretion without affecting glucagon secretion. The increase in the glucagon/insulin ratio, by stimulating glucose production in the liver, was probably responsible for the xylazine-induced hyperglycaemia.