We examined the chronic effects of MK954, a novel orally active angiotensin II receptor antagonist, on blood pressure and renal function in 8 patients with essential hypertension for 2-4 weeks. All patients, four men and four women, 48.0 +/- 15.3 year-old (mean +/- SD), were hospitalized and given normal sodium diet (NaCl 10 g/day). After a control period with placebo for one week, MK954 was administered orally at 8 AM every day. The initial dose of MK954 was 12.5 mg/day, then the dose was increased up to 100 mg/day until diastolic blood pressure fell below 90 mmHg. The average dose was 59.4 +/- 43.7 mg/day. Casual blood pressure in supine position decreased significantly from 161.0 +/- 6.6/95.0 +/- 3.5 mmHg to 145.8 +/- 8.1/83.3 +/- 3.7 mmHg without any change in pulse rate. Non-invasive ambulatory blood pressure monitoring revealed that once daily administration of MK954 lowered blood pressure for 24 hours but did not affect circadian rhythm or variability of blood pressure. Reduction of blood pressure was slightly greater during day time than during sleeping time. Unlike a peptide angiotensin II antagonist, there was no pressor action of MK954 as agonist. No significant alternations were observed in body weight, serum electrolytes, creatinine clearance, urine volume or urinary excretion of sodium. Plasma renin activity (PRA) rised significantly after MK954 treatment but plasma aldosterone concentration (PAC) did not change. The reasons why PAC was not reduced are unclear. The doses of MK954 employed in this study might be insufficient to inhibit adrenal angiotensin II receptor. In conclusion, MK954 has long acting hypotensive effect in essential hypertension without affecting renal function.