Using AT_1a receptor-deficient mice (Agtr1a-/-) and in vivo autoradiography, we tested the hypothesis that intracellular uptake of angiotensin II (Ang II) in the kidney and adrenals is primarily mediated by AT_1a receptors and the response is regulated by prevailing endogenous Ang II. Wild-type (Agtr1a+/+) and Agtr1a-/- mice were pretreated with captopril (25 mg/kg/day) or losartan (10 mg/kg/day) for 2 weeks before [¹²⁵I]-Val⁵-Ang II was infused for 60 min. Intracellular uptake of [¹²⁵I]-Val⁵-Ang II was determined by quantitative in vivo autoradiography after circulating [¹²⁵I]-Val⁵-Ang II was washed out. Compared with wild-types, basal intracellular Ang II levels were 65% lower in the kidney (p<0.001), but plasma Ang II levels were 3-folds higher in Agtr1a-/- mice (p<0.01). While plasma [¹²⁵I]-Val⁵-Ang II levels were similar, urinary excretion of [¹²⁵I]-Val⁵-Ang II was 4-folds higher in Agtr1a-/- mice (p<0.001). By contrast, intracellular [¹²⁵I]-Val⁵-Ang II levels were 80% lower in the kidney and adrenals of Agtr1a-/- mice (p<0.01). Captopril decreased endogenous plasma and renal Ang II levels (p<0.01), but it increased intracellular uptake of [¹²⁵I]-Val⁵-Ang II in the kidney and adrenals of both wild-type and Agtr1a-/- mice (p<0.01). Losartan largely blocked renal and adrenal uptake of [¹²⁵I]-Val⁵-Ang II in both wild-type and Agtr1a-/- mice. Thus 80% of intracellular Ang II uptake in the kidney and adrenals is mediated by AT_1a receptors, whereas AT_1b receptor- and other non receptor-mediated mechanisms account for 20% of the response. Our results suggest that AT_1a receptor-mediated uptake of extracellular Ang II may play a physiological role in the kidney and adrenals.