Atherosclerosis is a chronic inflammatory disease of the arterial wall involving inflammation, redox imbalance, and impaired cholesterol transport. A high level of trimethylamine-N-oxide (TMAO) produced by meat and fat metabolism are involved in atherosclerosis development, but the exact relationship with inflammation is not completely clear. The study aimed to identify a possible association between TMAO; atherosclerotic changes in the aortic root; oxidative stress; and inflammation quantified by highly sensitive C-reactive protein (hsCRP), tumor necrosis factor alpha (TNF-α), and interleukin-1beta (IL-1ß) levels. TMAO dihydrate was administered via gastric gavage to 20 male Wistar rats for 90 days; one separate group received vehicle. The TMAO-treated animals were divided into two groups: one group received a low dose of TMAO (20 mg/day) and the other group received a high dose of TMAO (40 mg/day). Malondialdehyde (MDA), proinflammatory markers - IL-1ß, TNF-α, and hsCRP, total cholesterol, high-density lipoprotein (HDL)-cholesterol, triglycerides, and glucose were assessed 30 and 90 days after TMAO administration. Additionally, conventional histopathology and immunohistochemistry for collagen I distribution were performed. MDA, hsCRP, TNF-α, and IL-1ß levels increased after 90 days of TMAO administration in conjunction with significant changes suggestive of incipient atherosclerosis and inflammation of the aortic root. The increase was higher in the group treated with 40 mg/day TMAO compared with the group treated with 20 mg/day TMAO. Additionally, blood levels of TMAO were significantly correlated with hsCRP, TNF-α, IL-1ß levels, but also with MDA, low HDL-cholesterol levels, and high triglyceride levels. The increase in MDA and inflammatory cytokines and modification of lipid metabolism markers may explain the pro-atherogenic effect of TMAO.