Plasma-renin activity was studied, under different conditions of sitmulation, in normal subjects and patients with essential hypertension. In normal subjects, three components of renin release, "basal," "neural," and "sodium-sensitive," could be delineated. Among the hypertensive patients, abnormal patterns of renin release were noted. The categorization of patients according to "plasma-renin status," however, differed with the conditions of testing, being dependent both on the pathophysiology of the essential hypertension and the character of the renin-releasing stimulus, specifically whether this stimulus elevated plasma-renin activity through a predominantly neural (upright posture) or non-neural mechanism (dietary sodium deprivation). Abnormal plasma-renin activity in essential hypertension resulted in part from disordered sympathetic nervous system function. Patients with mild hypertension and elevated plasma-renin activity exhibited increased neural stimulation of renin release, whereas in patients with low plasma-renin values, all three components of renin release, including the neural element, were diminished.