1. Intra-arterial pressure was recorded continuously in 26 patients with uncomplicated essential hypertension under standardized conditions. Pressure was analysed beat by beat by computer and variability measured as the standard deviation of the normally distributed frequency histogram.
2. Variability was strongly influenced by physical activity, being least during sleep and increasing progressively with bed rest and ambulation. Variability during daytime was not related to time.
3. Systolic variability correlated directly with systolic pressure. An independent inverse relationship with baroreflex sensitivity was observed. Systolic variability tended to increase with obesity.
4. None of the following were related independently with variability: age; race; sex; plasma renin activity; plasma angiotensin II; plasma noradrenaline; plasma adrenaline.