We examined circadian variations in blood pressure, pulse rate, and other physiological variables, including hormone levels, in 16 patients in a persistent vegetative state (mean age -/+ SE; 66.1 -+/ 3.9 yr). Cerebrovascular accident was responsible for brain damage in 12 (75%) of the 16 patients. Blood pressure was measured for 24 h with an ambulatory blood pressure monitoring device. We monitored the temperature of the urinary bladder and measured urinary excretion of epinephrine, norepinephrine, 17-hydroxycorticosteroids, water, and sodium. When data were analyzed by analysis of variance, significant circadian changes were observed in body temperature and urinary excretion of hormones and sodium, but not in blood pressure or pulse rate. Individual analysis of rhythmicity using the cosinor method detected small but significant circadian variations in blood pressure and pulse rate in five of six patients who showed a simple organized response to noxious external stimuli. The disappearance of variation in blood pressure in patients in a vegetative state appeared to be related, in part, to the lack of response to external stimuli. Our findings suggest that the circadian variation in blood pressure may largely depend on external environmental factors.