The abrupt cessation of clonidine therapy can induce a withdrawal syndrome. This may also appear immediately after anaesthesia if clonidine medication is discontinued during the operation day. We studied (1) whether the withdrawal syndrome occurs postoperatively as often as otherwise when clonidine therapy is discontinued, and (2) the action of this clonidine withdrawal on the circulation as compared to patients who received clonidine without interruption during the operation day. During the recovery period in 2 out of 10 patients in whom clonidine medication was discontinued, a hypertensive crisis occurred, which was relieved by clonidine readministration. Clonidine given with premedication eliminated high rises in blood pressure during anaesthesia. During the recovery period, the regular intramuscular administration of the same dose of clonidine as the patients had used orally decreased blood pressure to almost normotensive levels. The slightly increased urinary catecholamine excretion and plasma renin activity in these patients might, however, indicate that the circulatory homeostasis was disturbed in some degree. The results suggest that it is important to give clonidine continuously, even during the operation day. The dose may be the same as, or preferably somewhat smaller than that which the patients have previously received orally.