1. We have monitored intra-arterial pressure continuously throughout the day and analysed every beat by computer.
2. Baroreflex sensitivity (ms/mmHg) was measured four times a day (07.00, 12.00, 17.00 and 20.00 hours) in seven patients and three times a day (07.00, 12.00 and 17.00 hours) in 23 patients. The diurnal variation of baroreflex sensitivity in individual patients was 0–5.3 ms/mmHg and was not consistent. Mean baroreflex sensitivity was reduced in hypertensive patients compared with normotensive subjects.
3. A negative correlation was seen between baroreflex sensitivity and the variability of systolic blood pressure.
4. Baroreflex sensitivity was well correlated with the variability of heart rate.