Pregnant women who subsequently develop preeclampsia are highly sensitive to infused angiotensin (Ang) II; the sensitivity persists postpartum. Activating autoantibodies against the Ang II type 1 (AT₁) receptor are present in preeclampsia. In vitro and in vivo data suggest that they could be involved in the disease process. We generated and purified activating antibodies against the AT₁ receptor (AT₁-AB) by immunizing rabbits against the AFHYESQ epitope of the second extracellular loop, which is the binding epitope of endogenous activating autoantibodies against AT₁ from patients with preeclampsia. We then purified AT₁-AB using affinity chromatography with the AFHYESQ peptide. We were able to detect AT₁-AB both by ELISA and a functional bioassay. We then passively transferred AT₁-AB into pregnant rats, alone or combined with Ang II. AT₁-AB activated protein kinase C-α and extracellular-related kinase 1/2. Passive transfer of AT₁-AB alone or Ang II (435 ng/kg per minute) infused alone did not induce a preeclampsia-like syndrome in pregnant rats. However, the combination (AT₁-AB plus Ang II) induced hypertension, proteinuria, intrauterine growth retardation, and arteriolosclerosis in the uteroplacental unit. We next performed gene-array profiling of the uteroplacental unit and found that hypoxia-inducible factor 1α was upregulated by Ang II plus AT₁-AB, which we then confirmed by Western blotting in villous explants. Furthermore, endothelin 1 was upregulated in endothelial cells by Ang II plus AT₁-AB. We show that AT₁-AB induces Ang II sensitivity. Our mechanistic study supports the existence of an “autoimmune-activating receptor” that could contribute to Ang II sensitivity and possible to preeclampsia.