The blood flow rate in subcutaneous adipose tissue was measured on the lower legs of 11 patients with chronic lower-leg venous insufficiency and ulceration and in eight age-matched control subjects for 12-20 h, under ambulatory conditions, using the 133Xe wash-out technique with portable Cadmium telluride (CdTe(Cl)) detectors. In both groups, the change from an upright to a supine position at the beginning of the night period elicited an instantaneous increment in the blood flow rate of 30-40% with a decrease in the central and local postural sympathetic vasoconstrictor activity. After approximately 1 h of sleep, a considerable increase in blood flow rate was seen in both patient and control groups which persisted for nearly 100 min. In the patient group, the mean increase was 137% compared to a mean increase of 68% in the control group (P less than 0.01). The blood flow then returned to the same level as at the beginning of the night and remained stable until the subjects woke in the morning. The differences between this nocturnal hyperaemic phase and the adjacent phases were highly significant in both groups (P less than 0.0001 and P less than 0.005, respectively). The blood flow levels measured on the second day were of the same magnitude as those registered on the first day. The mean values of the wash-out rate constants (k) of the two groups were of the same magnitude in all phases of subcutaneous blood flow level, except in the hyperaemic phase, where it was significantly higher in the patient group (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)