Guanadrel sulfate, a new adrenergic neuron inhibitor similar to guanethidine sulfate, was tested on 199 outpatients by 11 investigators. The patients had mild, moderate, or severe hypertension as determined by diastolic blood pressures of 95 to 105, 106 to 114, and 115 to 120 mm Hg, respectively. Guanadrel was found to be an effective antihypertensive agent for all levels of hypertension. Since guanadrel has a short onset of action and a short offset of action, which prevents many of the side effects of guanathidine, the dosage could be adjusted rapidly and safely. At low doses side effects are infrequent. There was no organ toxicity and no CNS effect. Guanadrel should be an effective step II or step III drug for treatment of hypertension.