In this study, the effect of oral ciprofloxacin on the structure of the thoracic aorta in rats was investigated. Twenty four male albino rats were divided into 4 groups (6 rats/group): group I (adult control), group II (adult rats treated with ciprof loxacin), group III (senile control), and group IV (senile rats treated with ciprof loxacin). Rats in groups II and IV received ciprofloxacin via oral gavage in a daily dose of 3.5 mg/kg/d for 14 days, while control rats received equivalent amount of distilled water used to dissolve the drug. After 2 weeks, all rats were sacrificed, thoracic aortae were dissected, and half of the specimens were processed for paraffin sections and examined by light microscopy. The other half of the specimens were prepared for scanning electron microscopy. Sections from rats treated with ciprofloxacin showed evident damaging effect on aortic wall particularly in (group IV). Aortic intima showed, focal desquamation of the lining epithelium. Tunica media exhibited loss of the normal concentric arrangement and degeneration of the smooth muscle cells. Immune staining for alpha smooth muscle actin showed muscle damage. Interestingly, some sections in (group IV) showed out-pouch (aneurysm like) of the aortic wall. There was dense collagen fibers deposition. Scanning electron microscopic observations of (group IV) revealed uneven intima, adherent blood cells and fibrin filaments to damaged intima, and out-pouch formation. It was concluded that oral ciprofloxacin caused deleterious structural changes in the thoracic aortic wall of rats explaining clinical observations of fluoroquinolones induced risk of aortic dissection and aneurysm.