A total of 18 diurnally active subjects with uncomplicated, mild to moderate, essential hypertension were studied to compare the efficacy of the morning versus evening administration of an oral olmesartan medication. After a two-week, wash-out/placebo run-in period, subjects with clinic diastolic blood pressure (DBP) > or = 90 mm Hg and <110 mm Hg began 12 weeks of 20 mg olmesartan medoxomil tablet therapy at 08:00 h daily. Four of the 18 subjects required dose escalation to 40 mg at eight weeks because of clinic DBP > or = 90 mm Hg. After the 12-week period of once-a-day 08:00 h treatment, subjects were immediately switched to an evening (20:00 h) drug-ingestion schedule for another 12-week period without change in dose. Subjects underwent 24 h ambulatory blood pressure monitoring (ABPM) before the initiation of morning treatment and at the end of both the 12-week morning and evening treatment arms. Dosing time did not exert statistically significant differences on the efficacy of olmesartan: the reduction from baseline in the 24 h mean systolic (SBP) and DBP was, respectively, 18.8 and 14.6 mm Hg with morning dosing and 16.1 and 13.2 mm Hg with evening dosing (p>0.152 between groups). The amplitude of the BP 24 h pattern did not vary with dosing time, indicating full 24 h BP reduction no matter the clock hour of treatment. Although, the BP-lowering effect was somewhat better with morning dosing, the results of this study suggest that the studied olmesartan medoxomil preparation efficiently reduces BP when ingested in the morning (08:00 h) or evening (20:00 h) in equivalent manner, based on statistical testing, throughout the 24 h.