Objective: To investigate the expression of endothelial nitric oxide synthase (eNOS) in human cardiac tissues. Design: We attempted to determine the genetic expression and localization of eNOS in normal human atrial tissue. Material and Methods: In normal human right atrial tissues from five donors during cardiac transplantation, eNOS expression and localization were assessed by using Northern blot analysis, in situ hybridization, and immunohistochemical staining. Results: Northern blot analysis and in situ hybridization demonstrated that eNOS messenger RNA is present in cardiomyocytes. Positive immunohistochemical staining was observed in the cytoplasm of cardiomyocytes. Conclusion: These studies show for the first time the genetic expression and distribution of eNOS in human atrial myocardium and suggest that the eNOS-mediated paracrine and autocrine pathway may participate in the control of myocardial function in humans.