Nitric oxide (NO) plays an important role in the regulation of blood flow. Pharmacological tools and a series of other techniques have been developed for studying the NO/ L-arginine pathway, but it has proved difficult to make a quantitative link between effect and tissue NO concentration. NO microsensors have been applied with success for the measurement of NO in suspensions of mitochondria and cells, such as platelets and leukocytes, and in cell cultures, which together with other interventions or measurements are particularly useful for the examination of cell signalling related to the NO/ L-arginine pathway. In isolated vascular segments, studies using the NO microsensor have defined the relationship between NO concentration and relaxation and revealed residual NO release in the presence of NO synthase inhibitors. Moreover, simultaneous measurements of NO concentration and vasorelaxation in isometric preparations have shown that agonist-induced relaxation is L-arginine dependent and NO release is reduced in hypertension. By placing NO microsensors in catheters, it is possible to measure NO in the living animal and man. This approach has been applied for the measurements of NO concentration in relation to increases in flow, erection, in conditions of hypoxia, and in endotoxemia. However, further methodological development of NO microsensors is necessary to avoid the influence of changes in temperature, pH and oxygen on the measurements.