In adult mice we determined whether expression of angiotensin II (Ang II) type 1A receptors (AT(1A)Rs) in C1 neurons mediates the excitation of the rostral ventrolateral medulla (RVLM) by Ang II. Blood pressure, heart rate, and sympathetic nerve activity were measured in anesthetized, artificially ventilated wild-type (n=15) and AT(1A)R knockout (AT(1A)(-/-); n=9) mice. Microinjection of Ang II (50 nL of 0.1 to 1.0 mmol/L) into the RVLM induced a dose-related, sympathetically mediated pressor response (maximum of 17+/-2 mm Hg) in wild-type mice. These microinjections had no effect in AT(1A)(-/-) mice. Endogenous AT(1)Rs occur on catecholaminergic C1 neurons in the RVLM. We induced AT(1A)R or green fluorescent protein expression in C1 neurons of AT(1A)(-/-) mice through bilateral microinjection of replication-deficient lentiviruses, with transgene expression under the control of a phox2 transcription factor binding promoter (PRSx8) (Lv-PRSx8-AT(1A), n=10, and Lv-PRSx8-GFP, n=5). Transgene expression was observed in a significant proportion of RVLM C1 neurons. In anesthetized Lv-PRSx8-AT(1A) injected mice, unilateral RVLM microinjection of Ang II (50 nL of 1 mmol/L) increased blood pressure (17+/-4 mm Hg) and sympathetic nerve activity (155+/-32%). No response to Ang II occurred in Lv-PRSx8-GFP microinjected mice. These results show that Ang II-mediated excitation of RVLM neurons in adult mice depends on the AT(1A)R with little or no effect of type 1B or 2 receptors. Expression of the AT(1A)R predominantly in C1 catecholamine neurons restores the response to Ang II in the AT(1A)(-/-) mouse and demonstrates that these neurons are sympathoexcitatory in the mouse.