Serotonin (5-HT) has been implicated in cerebral aneurysm rupture, but it is unclear whether 5-HT plays a role in aortic aneurysm development and rupture, despite well known contractile effects of 5-HT through aortic 5-HT receptors. Abdominal aortic aneurysms (AAAs) induced by angiotensin II (AngII) infusion to mice exhibit periaortic inflammation and are prone to rupture. Periaortic fat (PAF), a potential source of 5-HT through tryptophan hydroxylase 1 (Tph1), has been implicated in AAA development. We quantified mRNA abundance of 5-HT receptors (Htr1b, Htr2a, Htr2b, Htr3a, and Htr7) and Tph1 in thoracic and abdominal aortas and surrounding PAF. Compared with other 5-HT receptors, we detected high levels of serotonin 3 receptor type a (Htr3a) mRNA in the abdominal aortas and abdominal PAF. Tph1 mRNA and 5-HT immunostaining were detected in aortas and PAF, with 5-HT levels higher in abdominal than thoracic PAF, and higher in epididymal white than interscapular brown fat. AngII infusion facilitated evoked [