The demonstration of 5'-nucleotidase in neural tissue is achieved at both the light and electron microscopic levels by means of an enzyme cytochemical lead method, which is specific, sensitive and fast. By its activity this adenosine-producing ecto-enzyme (EC 3.1.3.5) outlines cellular surface membranes at the ultrastructural level. It is classically known as a marker of myelin and of astrocytes as well as (activated) microglial cells in the mature nervous system. In recent years, we discovered that 5'-nucleotidase is transiently active within synaptic clefts under conditions of development and regeneration. The enzyme is also seen at terminals in the mature retina and olfactory bulb, where spontaneous synaptic turnover occurs at adulthood. Thus, 5'-nucleotidase cytochemistry is useful in revealing sites of glial reactions and synaptic plasticity in CNS development and repair. It is assumed that the molecule affects terminal formation and cell motility due to dual functions in adenosine production and cell adhesion. Finally, at the light microscopic level, 5'-nucleotidase activity displays a dense neuropil staining which identifies topographic sub-units of certain parts of the nervous system, such as the striosomes of the basal ganglia, ocular dominance columns of the visual cortex and parasagittal bands of the cerebellum.