Clonidine hydrochloride is a new antihypertensive agent with a primary site of action in the central nervous system. When administered with a diuretic, it is effective for long-term therapy and may be particularly useful in patients with moderately severe hypertension. Clonidine is comparable to methyldopa in efficacy but may cause side-effects more frequently. The only potentially serious adverse reaction that has been reported is a rebound increase in blood pressure that may occur following rapid withdrawal.