Objectives: To test the hypothesis that Angiotensin II (Ang II) is a contributing factor to the response pattern in African Americans (AAs) who retain rather than excrete sodium during mental stress. Design/Study Participants: Double-blind, randomized, cross-over trial of 87 healthy AAs aged 18 to 50 years. Interventions: The study participants received either a placebo or irbesartan, (150 mg PO), an Ang II receptor antagonist, for seven days prior to stress testing. Urinary sodium excretion (UNaV) and systolic blood pressure (SBP) were collected prior to and throughout a mental stress protocol (rest and stress period). Setting: A southeastern university. Main Outcome Measures: Ang II, SBP, and sodium retention. Results: During the placebo condition, 62 participants showed the expected increase in UNaV (excreters) while 25 participants reduced UNaV during stress (retainers). Irbesartan retainers demonstrated a reversal in the direction of their natriuretic response, now increasing UNaV in response to stress (∆ UNaV of -.094 mmol/min with placebo vs .052 mmol/min on irbesartan; P<.001). In excreters, irbesartan reduced SBP levels during both rest (-2.36 mm Hg; P=.03) and stress (-4.59;P<.0001), and an even more pronounced reduction in SBP was demonstrated by retainers on treatment during both rest (-4.29 mm Hg; P=.03) and stress (-6.12; P<.001). Conclusions: Ang II contributes to sodium retention in retainers. Furthermore, our findings indicate that suppression of Ang II has a beneficial effect on SBP during rest and stress in this population.