We studied the hemodynamic, neurohumoral, and biochemical effects of the novel angiotensin type 1 (AT1) receptor antagonist irbesartan in 86 untreated patients with essential hypertension on a normal sodium diet. According to a double-blind parallel group trial, patients were randomized to a once-daily oral dose of the AT1receptor antagonist (1, 25, or 100 mg) or placebo after a placebo run-in period of 3 weeks. Randomization medication was given for 1 week. Compared with placebo, 24-hour ambulatory blood pressure did not change with the 1-mg dose, and it fell (mean and 95% confidence interval) by 7.0 (4.2-9.8)/6.1 (3.9-8.1) mm Hg with the 25-mg dose and by 12.1 (8.1-16.2)/7.2 (4.9-9.4) mm Hg with the 100-mg dose. Heart rate did not change during either dose. With the 25-mg dose, the antihypertensive effect was attenuated during the second half of the recording, and with the 100-mg dose, it was maintained for 24 hours. Baseline values of renin and the antihypertensive response to the 25- and 100-mg doses were well correlated (r = .68, P < .01). Renin did not change with the 1-mg dose, but it rose threefold to fourfold with the 25-mg dose and fourfold to fivefold with the 100-mg dose 4 to 6 hours after administration. With the 100-mg dose, renin was still elevated twofold 24 hours after dosing. The changes in renin induced by the AT (1) receptor antagonist were associated with parallel increments in angiotensin I and angiotensin II. Aldosterone, despite AT1receptor blockade, did not fall. Compared with baseline values, plasma norepinephrine increased moderately with the 100-mg but not with 25- or 1-mg dose. Serum uric acid and its 24-hour urinary excretion did not change. In conclusion, in essential hypertension, once-daily irbesartan effectively lowers blood pressure. This effect is maintained for 24 hours with a 100-mg dose. Unlike the AT1receptor antagonist losartan, irbesartan exerts no uricosuric effect, suggesting that this is an effect unrelated to AT1receptor blockade. (Hypertension. 1995;25:22-29.)