Abstract: Objective: To investigate the relationship between rennin-angiotensin system and circadian rhythm of blood pressure in hypertensive patients with nocturnal hypertension. Methods: Ambulatory blood pressure monitoring （ABPM） was performed over a 24-hour period both in 30 normotensive subjects and 30 hypertensive patients with nocturnal hypertension. In the same period, plasma rennin activity and angiotensin II level were assayed by radioimmunoassay method in all of them at 8∶00, 16∶00, 22∶00 and 2∶00 respectively. Results: The curves of the circadian variation of the blood pressure in normotensive subjects and hypertensive patients were similar and presented two peaks and one trough. The plasma rennin activity in patients with PRA higher than normotensive subjects reached highest at 8∶00 and lowest at 2∶00 and was positively correlated with mean arterial pressure（P＜0.05）. In hypertensive patients, the plasma angiotensin II levels at 8∶00, 16∶00, 22∶00 and 2: 00 were all higher than normotensive subjects and positively correlated with mean arterial pressure（P＜0.05）, especially with systolic blood pressure（P＜0.01）. The plasma angiotensin II levels in hypertensive patients and normotensive subjects demonstrated same circadian variation, which had the highest value at 8∶00 and lowest value at 2∶00. Conclusion: The plasma rennin-angiotensin system may play important role in the pathogenesis of nocturnal hypertension and the regulation of circadian variation of blood pressure in hypertensive patients.