This study analysed the effects of both seven and 30 days of treatment with clonidine on heart rate, blood pressure (BP), plasma norepinephrine (NE) levels and dopamine-beta-hydroxylase (DBH) activity at rest and after standing in seven patients with essential hypertension. The patients and 10 age-matched normotensive control subjects were evaluated under medication-free baseline conditions; the hypertensive patients were then given clonidine 0.1 mg twice daily. Baseline plasma NE and DBH levels were similar between the groups. Seven and 30 days of clonidine therapy reduced blood pressure (p less than 0.001) and NE concentrations (p less than 0.05) significantly. Clonidine did not effect the percent increase in NE induced by standing and was not associated with orthostatic hypotension. The blood pressure lowering effects of clonidine appear related to the centrally mediated and/or direct suppression of peripheral noradrenergic activity, indicating the utility of clonidine in cases of hypertension were the sympathetic nervous system (SNS) is hyperactive. Clonidine may also prove especially useful in cases where the initial pharmacotherapy (such as a diuretic) causes activation of the SNS.