Angiotensin (ANG) type 1A (AT_1A) receptor-null (AT_1A^−/−) mice exhibit reduced afferent arteriolar (AA) constrictor responses to ANG II compared with wild-type (WT) mice, whereas efferent arteriolar (EA) responses are absent (Harrison-Bernard LM, Cook AK, Oliverio MI, and Coffman TM. Am J Physiol Renal Physiol 284: F538–F545, 2003). In the present study, the renal arteriolar constrictor responses to norepinephrine (NE) and/or ANG II were determined in blood-perfused juxtamedullary nephrons from kidneys of AT_1A^−/−, AT_1B receptor-null (AT_1B^−/−), and WT mice. Baseline AA diameter in AT_1A^−/− mice was not different from that in WT mice (13.1 ± 0.9 and 12.6 ± 0.9 μm, n = 7 and 8, respectively); however, EA diameters were significantly larger (17.3 ± 1.4 vs. 11.7 ± 0.4 μm, n = 10 and 8) in AT_1A^−/− than in WT mice. Constriction of AA (−40 ± 8 and −51 ± 6% at 1 μM NE) and EA (−29 ± 6 and −38 ± 3% at 1 μM NE) in response to 0.1–1 μM NE was similar in AT_1A^−/− and WT mice. Baseline diameters of AA (13.5 ± 0.7 and 14.2 ± 0.9 μm, n = 9 and 10) and EA (15.4 ± 1.0 and 15.0 ± 0.7 μm, n = 11 and 9) and ANG II (0.1–10 nM) constrictor responses of AA (−25 ± 4 and −31 ± 5% at 10 nM) and EA (−32 ± 6 and −35 ± 7% at 10 nM) were similar in AT_1B^−/− and WT mice, respectively. ANG II-induced constrictions were eliminated by AT₁ receptor blockade with 4 μM candesartan. Taken together, our data demonstrate that AA and EA responses to NE are unaltered in the absence of AT_1A receptors, and ANG II responses remain intact in the absence of AT_1B receptors. Therefore, we conclude that AT_1A and AT_1B receptors are functionally expressed on the AA, whereas the EA exclusively expresses the AT_1A receptor.