Sleeping and awake blood pressures were related to sleep oxygenation and awake control of breathing in hypertensive and normotensive older men. During sleep, episodes of both hypertension and hypotension were observed, but hypotension was prominent. Five of 26 subjects exhibited episodes with a minimum mean arterial pressure of 60 mm Hg or less. These hypotensive episodes were associated with hemoglobin desaturation below 80 percent, secondary to sleep-related breathing disorders, and elevated supine nasopharyngeal airway resistance. Hypotensive subjects were habitual snorers. Waking hypertension was not associated with sleep-related breathing disorders in this older sample. In some older persons with sleep-related breathing disorders, sympathetic reflexes may be impaired, permitting hypotension and risk of circulatory failure. Epidemiologic evidence supports the hypothesis that this mechanism can explain the elevated incidence of cerebral infarction during sleep.