SUMMARY Infra-arterial pressure was recorded continuously in 26 patients with uncomplicated essential hypertension under standardized conditions. Recordings were analyzed beat by beat to obtain mean pressures and variability, expressed as tbe standard deflation of the frequency histogram. The major factors influencing variability were tbe level of pressure and tbe Intensity of physical activity; systolic variability increased with progressive impairment of sino-aortic baroreflexes. Diastollc pressure increased with the level of sympathetic activity as reflected by plasma noreplnephrine levels. After allowance for the decrease of plasma renin activity (PRA) with age, direct relationships were observed between PRA (log values) and the level of pressure and systolic variability; plasma angiotensin II values did not correlate. Systolic variability increased with the systolic response to cold but was unrelated to the response to dynamic or isometric exercise. Variability also tended to increase with obesity and was unrelated to age, sex, or race.