Evidence from several laboratories strongly suggests that brain capillaries play a dynamic role in the regulation of the internal environment of the brain and, further, that these activities are under neuroendocrine control. First, brain capillaries exhibit a number of anatomical and biochemical features unique to membranes known to regulate water and electrolyte permeability. These include tight junctions between endothelial cells, a high mitochondrial content and a unique compliment of intracellular enzymes. Second, brain capillaries appear to be functionally innervated by adrenergic neurons originating in brain. Finally, brain capillaries exhibit several functional characteristics of membranes that dynamically regulate water and electrolyte permeabilities. These include a restricted permeability to water, a prompt and reversible increase in water permeability to transient hyperosmolarity, a prompt and reversible decrease in permeability to increase perfusion pressure, a change in permeability associated with activation of the central as well as peripheral sympathetic nervous system, and an increase in permeability to centrally administered vasopressin as well as angiotensin II.