Heart failure is a common problem associated with considerable mortality and morbidity. The mechanisms underlying the heart failure syndrome, which remain poorly understood, may involve an inflammatory process. Nitric oxide (NO) and various cytokines could play an important role in this inflammatory process. Recent evidence has emerged in both animal models and humans suggesting that both of these mediators may play an important role in heart failure. NO is synthesized by the NO synthase family of enzymes. Two of these enzymes are constitutive, endothelial NO synthase and neuronal NO synthase. The third enzyme, inducible NO synthase, is capable of producing large amounts of NO once induced by mediators such as interleukin (IL)-1, IL-2, IL-6, tumour necrosis factor (TNF)-alpha, and interferon-gamma. Endothelial NO synthase is present in the heart in the endocardium, cardiac myocytes, and cardiac conduction tissue. Inducible NO synthase is present in cardiac myocytes, endocardium, vascular smooth muscle cells, and infiltrating inflammatory cells. Evidence from both animal models and patients suggests that NO exerts a negative inotropic effect. Increased inducible NO synthase, TNF-alpha, and IL-6 have been found in patients with heart failure in several studies. In other studies, decreased endothelial NO synthase was found in patients with heart failure. TNF-alpha and IL-6 may be produced in heart failure and may induce inducible NO synthase, resulting in NO production, which acts as a negative inotrope. Endothelial NO synthase may be decreased as a result of downregulation by TNF-alpha or inducible NO synthase. The possible role of these mediators in heart failure needs further evaluation because these findings could have novel therapeutic implications.