The objective of this study was to determine the effects and mechanisms of serum amyloid A (SAA) on coronary endothelial function. Porcine coronary arteries and human coronary arterial endothelial cells (HCAEC) were treated with SAA (0, 1, 10 or 25 µg/ml). Vasomotor reactivity was studied using myograph tension system. SAA significantly reduced endothelium-dependent vasorelaxation of porcine coronary arteries in response to bradykinin in a concentration-dependent manner. SAA significantly decreased endothelial nitric oxide synthase (eNOS) mRNA and protein levels as well as NO bioavailability, while increased reactive oxygen species (ROS) in both artery rings and HCAECs. In addition, the activities of internal antioxidant enzymes catalase and superoxide dismutase were decreased in SAA-treated HCAECs. Bio-plex immunoassay analysis showed the activation of JNK, ERK2, and IB after SAA treatment. Consequently, antioxidants seleno-L-methionine and MnTBAP and specific inhibitors for JNK and ERK1/2 effectively blocked SAA-induced eNOS mRNA decrease and SAA-induced decrease in endothelium-dependent vasorelaxation in porcine coronary arteries. Thus, SAA at clinically relevant concentrations causes endothelial dysfunction in both porcine coronary arteries and HCAECs through molecular mechanisms involving eNOS downregulation, oxidative stress, and activation of JNK and ERK1/2 as well as NF-B. These findings suggest that SAA may contribute to the progress of coronary artery disease.