In order to study blood pressure variability we have measured blood pressure, heart rate, plasma noradrenaline and adrenaline concentration and plasma renin activity during sleep and during the waking process in 20 subjects with borderline hypertension. The responses to a number of standardized tests were also measured. These were reading, mental arithmetic, change in posture, physical exercise and the response to intravenously injected phenylephrine and noradrenaline. The sensitivity of the baroreflex for heart rate control was also determined from the relationship between heart period (R-R interval) and change in systolic pressure after the injection of phenylephrine. Blood pressure was also recorded continuously for 24 hours. From the lowest levels achieved during sleep, blood pressure rose as the subjects regained consciousness but was not restored to its baseline value until mental activity was also restored by reading. Blood pressure rose further with mental arithmetic. These changes were accompanied by a greater proportional rise in plasma adrenaline than in plasma noradrenaline concentrations. Plasma renin activity changed little. The pressor responses to phenylephrine and noradrenaline were inversely related to baroreceptor sensitivity. The fall in blood pressure with sleep and the rise with mental arithmetic was also inversely related to the sensitivity of the reflex. Systolic pressure recorded throughout the day was inversely related to the R-R interval in each subject. The slope of this relationship and range of R-R interval was greatest in the subjects with the most sensitive baroreflexes.