In mice, angiotensin (Ang) II type-1 (AT(1)) receptors exist as AT1a, and AT(1b) subtypes. In an effort to understand the role of AT(1b) in regulating vascular function, AT(1) subtype mRNA and its functional relevance in the mesenteric resistance vessels were determined using wildtype (WT) and AT(1a) knockout (AT(1a)(-/-) mice) mice. With RT-PCR followed by restriction-enzyme digestion, we found that AT(1b) accounted for almost all (98%) of AT(1) receptors in the mesenteric resistance vessels of WT mice. Also, the Ang II response in the vessels of AT(1a)(-/-) mice was comparable to that of WT mice, suggesting an important role for AT(1b) in regulating vasoreactivity. To further characterize AT(1b) receptor distribution, several other tissues were examined. Among them, AT(1b) is only predominantly expressed in hypothalamus, whereas AT(1a) exists exclusively or as a major subtype in heart, pituitary, adrenal glands and brainstem. These results further underscore a tissue-specific role for AT(1b) receptor in mice.