The localization of non-specific alkaline phosphatase and cholinesterase activities evidenced in capillaries endothelium of the Rat brain have been compared in cerebral areas with or without the blood-brain barrier (BBB). There is a good topographical correlation: structures devoid of BBB (caudal part of the subfornical organ, choroid plexus, area postrema) do not display any non-specific cholinesterase activity with photonic microscope methods and an intense alkaline phosphatase activity; whereas structures with BBB (cortex, subcommissural organ, rostral part of the subfornical organ) show both enzymatic activities. At the ultrastructural level, more sensitive methods allow to disclose a non-specific cholinesterase activity in the reticulum endoplasmic cisternae and in the nuclear membrane in all endothelial cells. Alkaline phosphatase activities are present in the capillaries endothelium and in the conjonctive cells of pericapillaries spaces which exist in the circumventricular organs. Besides, these activities are closely related with the lateral and basal plasmalemmas of the choroidal cells responsible for the blood-cerebro-spinal fluid barrier. The apical plasmalemma is almost devoid of enzymatic activity. This result confirms the noticeable polarity of the choroidal cells, both morphologically and cytochemically.