Treatment was interrupted abruptly in 6 hypertensive patients receiving clonidine 0·45-5·4 mg daily. Blood-pressure rose to pretreatment levels within 24-48 h of withdrawal and was accompanied by insomnia, headache, flushing, sweating, and apprehension. These symptoms began 18-20 h after the last dose of clonidine. Plasma-noradrenaline levels and urinary catecholamine excretion increased 24-72 h after withdrawal of clonidine. The subjective symptoms were most prominent in patients on higher doses (>1 mg/day) and in those who had previously been receiving treatment with other antihypertensive drugs. One patient on a very low daily dose (0·15 mg) of clonidine had no symptoms and no significant changes in blood-pressure or catecholamine production after drug withdrawal.