Under normal sleep-wake conditions, noradrenaline (NA) secretions in supine subjects exhibit a weak circadian variation with a peak that occurs around noon; the sleep span is characterized by reduced NA secretion. Some investigators have reported that the circadian NA rhythm is completely obliterated during sleep deprivation. In our laboratory, plasma NA was assayed every hour for 24 h in nine healthy men 20-23 years of age. All men were deprived of sleep and were required to eat and walk around every hour to prevent sleep. However, subjects remained supine for 20 min before blood samples were collected to eliminate the effect of activity. Persistence of a slight decrease in the night concentration in several subjects, despite sleep deprivation, suggests that NA secretion may be influenced by a biological clock whose activity becomes visible when the influence of posture is removed.