1. We have recorded ambulant intra-arterial blood pressure in five normo- and 14 hyper-tensive subjects over 24 h, particular attention being paid to the timing of arousal.

2. The data were edited beat-by-beat before storing systolic, diastolic, and mean blood pressure ± sd for every 2 min period on a computer file.

3. When pooled hourly mean data are plotted there is a rise of 52·9 mmHg systolic and 22·3 mmHg diastolic pressure from 03.00 hours to 11.00 hours in the 14 hypertensive subjects.

4. When the data are plotted in 20 min periods for the hours before and after arousal it is clear that the major rise in pressure occurs abruptly at that time. The 2 h period before arousal shows only a small rise in pressure, i.e. 6·9 mmHg systolic and 2·9 mmHg diastolic.

5. It is suggested that the apparent rapid early morning rise of blood pressure is an artifact of the method of analysis.

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