Twenty-seven healthy people with a wide range of casual blood pressure readings (120/70 to 230/120 mm. Hg) had their blood pressure recorded continuously over a 24-hour period. Arterial pressure fell during sleep and only rose appreciably after the patients had awakened and a further rise was associated with physical activity, once they were out of bed. This pattern was observed in 18 untreated patients and in nine currently receiving hypotensive treatment. This study suggests that arterial pressure is not specifically related to time and is chiefly governed by physical activity and sleep.