INTRODUCTION In normal subjects and most essential hypertensives, circadian fluctuation of blood pressure (BP) has been well established [1,2]. The regulation of autonomic function over a 24-hour period is a result of feedback of circadian spontaneous rhythm. The neurohormonal system controls internal and external disturbances of the feedback mechanisms of regulative functions [3,4]. Evaluation of the circadian rhythm of blood pressure is particularly important in assessing the effects of treatment of hypertension and in studying the mechanisms of hypertension. Ambulatory blood pressure monitoring has been proved useful in terms of characterization of blood pressure profiles in normotensive and hypertensive subjects, evaluation of patients with mild or labile hypertension, degree of blood pressure elevation, prognosis of hypertension, and assessment of hypertension management. The prognostic implications of M Taiwan J Med 1998;1:27-33 Diurna l Blood Pressure Changes in Primary Aldosteronism - A Study of Ambulatory Blood Pressure Monitoring and Neurohormonal Changes Pei-Ying Pai, Hsiang-Tai Chou, Shi-Sheng Tsou, Ching-Chu Chen 1 , and Ting-Fu Wang Division of Cardiology, and 1 Endocrinology, Department of Internal Medicine, China Medical College Hospital, Taichung, Taiwan, R.O.C. The study was aimed to investigate the relationship between the 24-hour blood pressure and neurohormonal change in patients with secondary hypertension. Circadian blood pressure variation was studied in patients with chronic renal failure and primary aldosteronism. Ambulatory blood pressure was monitored every 10 minutes during daytime and every 30 minutes during nighttime. The daytime and nighttime systolic/diastolic blood pressures (SBP/DBP) in patients with primary hyperaldosteronism were significantly higher than those with primary hypertension (SBP:147 16 vs 125 11 mmHg, daytime; 142 23 vs 118 13 mmHg, nighttime; DBP: 96 18 vs 85 8 mmHg, daytime; 91 24 vs 81 9 mmHg, nighttime). In the renal form of secondary hypertension, the blood pressure did not reveal a significant difference with diurnal change in comparison with primary hypertension. The reduction in nocturnal blood pressure was less in primary hyperaldosteronism than in primary hypertension. Insufficient decrease of blood pressure during nighttime may warrant further investigation to diagnose secondary hypertension. Key words ambulatory BP monitor secondary hypertension neurohormonal Received: November 14, 1997 Revised: December 30, 1997 Accepted: January 15, 1998 Address reprint requests to: Pei-Ying Pai, Division of Cardiology, Department of Internal Medicine, China Medical College Hospital, No 2,Yuh-Der Road, Taichung, Taiwan R.O.C.