Background:A less than 10% decline in blood pressure (BP) during the night is known as a nondipping BP pattern. Nondipping BP has been shown to be associated with target organ damage and poorer cardiovascular outcomes. It was suggested that the angiotensin II receptor blocker could improve nocturnal dipping by enhancing daytime sodium excretion. In this study, we elucidated the effects of ARB and CCB on 24h ambulatory BP monitoring (ABPM) in untreated hypertensives. Method: In this study, fifty patients with untreated HTN (M:F =30:20; mean age 50.82 짹 13.70 years; the mean duration of medication 344 days), were examined by 24 ABPM before and after antihypertensive medication. We divided those patients into two groups [Group 1, n=20, treated with CCBs (M: F=9:11; mean age= 53 짹 15 years) and Group 2, n=30, treated with ARBs (M:F=21:9; mean age=49 짹 13 years)]. We compared �� dipping degree, [dipping degree (%) after medication] – [dipping degree (%) before medication] in SBP, �� dipping degree in DBP and �� morning surge, [morning surge (mmHg) after medication] – [morning surge (mmHg) before medication] between two groups. Result: There were no significant differences in comparison of clinical characteristics between two groups. The nocturnal dipping in SBP and DBP was significantlly improved more in Group 2 than in group 1; �� dipping degree in SBP between group1 and group 2 was -1.46 짹 7.44 vs. 4.13 짹 7.99 %, respectively (p=0.018) and �� dipping degree in DBP, -2.81 짹 7.78 vs. 5.11 짹 8.64 %, respectively (p=0.002). The �� morning surge was decreased in group 1 compared with group 2, but did not reach the statistical significance (-5.74 짹 18.90 vs. 5.25 짹 18.09 mmHg, p=0.051). Conclusions: ARB can improve effectively nocturnal dipping, but not morning surge in untreated hypertension.