OBJECTIVE AND METHODS: To determine correlations among ambulatory renin activity, ambulatory blood pressure and microalbumin excretion rate, 66 Korean essential hypertensives were studied after 4 week wash-out period. The ambulatory blood pressure was monitored every 30 minutes and mean BP were calculated automatically. Urinary microalbumin excretion rate(UAER) and ambulatory plasma renin activity(aPRA) collected at mid-day were measured by radioimmunoassays. Subjectives were divided into 2 groups by aPRA value(2ng/ml/hr). RESULT: 14 cases were high renin group and 52 cases low renin group. The mean BP were 148.83/94.69mmHg in low renin group, and 146.57/98.07mmHg in high-renin group without difference. UAER were not different also between both groups. 23.07%(4/14) of non-dippers were included in high renin group and 25.58%(12/52) in low renin group without statistical difference. The aPRA was significantly related to UAER and systolic and diastolic mean blood pressure. Also UAER was related significantly to day mean blood pressures. CONCLUSION: Thus aPRA is thought to be a meaningful indicator to predict hypertensive renal target organ damage as well as blood pressure measured with 24-hr ABPM.