1. Chronic two-kidney, one-clip hypertensive rats were infused with captopril for 5 days and the daily variability of blood pressure compared with that for both hypertensive rats infused with glucose and normotensive animals.

2. Blood pressure was measured continuously using a computer data collecting system.

3. Normotensive animals showed a stable level of arterial pressure throughout each 24 h period with troughs occurring when they slept during the daytime.

4. Hypertensive animals given glucose had an enhanced diurnal rhythm of blood pressure compared with normotensive rats, with peaks occurring during periods of activity at night as well as troughs when they slept during the day.

5. Hypertensive rats given captopril retained this enhanced pressure variability, in spite of the fact that blood pressure was significantly lower and angiotensin II was suppressed.

6. These results suggest that angiotensin II is not involved in the increased blood pressure variability of renal hypertension and that some other irreversible mechanism is responsible.

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