Nitric oxide derived from the vascular endothelium and other cells of the cardiovascular system has important roles in physiological regulation of blood flow and may have pathophysiological functions in cardiovascular disease. Nitric oxide can be synthesised from L-arginine by any of three isoforms of nitric oxide synthase (NOS), and its interaction with prostacyclin, its proposed mechanisms of action and cytotoxicity are briefly reviewed in the context of cardiovascular function. Although nitric oxide can hyperpolarize vascular smooth muscle, activation of the endothelium can induce hyperpolarization and vasodilatation by other means. Nitric oxide has important roles in the physiological regulation of local blood flow and blood pressure, especially during exercise and in response to shear stresses and other local factors in arterioles. Nitric oxide is also involved in neurogenic control of the microcirculation through autonomic efferent nerves and it contributes to vasodilatation and inflammation associated with activation of sensory nerves. In pathological circumstances, excess nitric oxide produced by inducible NOS compromises circulatory function in septic shock, during transplant rejection, and during myocardial ischaemia and reperfusion injury. Immunosuppressant drugs like cyclosporin A inhibit the expression of NOS through complex intracellular intermediates. Disturbances in the activity of constitutive and inducible NOS in the artery wall accompany the development of atherosclerosis, vasospasm and thrombosis, and may contribute to some forms of hypertension and diabetic vascular disease. Reversing the nitric oxide defect with therapeutic agents including angiotensin-converting enzyme inhibitors offers promise in protecting against some manifestations of vascular disease.