The centrally-acting antihypertensive drug guanfacine was studied in a group of 11 moderate hypertensives. In doses of 2 mg daily, an average reduction in diastolic blood pressure of 10.8 mmHg was achieved. Side-effects were few when doses were maintained below 3 mg daily. The blood pressure reduction was associated with a fall in plasma renin activity and an average weight gain of 1.8 kg. When guanfacine was tried in 6 very severe hypertensives who had proved resistant to other antihypertensive drugs, a similar reduction in diastolic pressure of 7 mmHg was achieved using a dose of 3 mg daily. It is considered that guanfacine is a useful new antihypertensive drug, effective in mild hypertension, and side-effects are few if doses are maintained below 3 mg daily. Above this dose, side-effects became prominent, and these included sedation, dry mouth and constipation.