Free myo-inositol (inositol) transport into the cerebrospinal fluid (CSF), brain, and choroid plexus and out of the cerebrospinal fluid was measured in rabbits. In vivo, inositol transport from blood into choroid plexus, CSF, and brain was saturable with an apparent affinity constant (K-t) of approximately 0.1 mM. The relative turnover of free inositol in choroid plexus (16 percent/h) was higher than in CSF 4percent/h) and brain (0.3percent/h) when meausred by tissue penetration of tracer [3-H]-labeled inositol injected into blood. However, the passage of tracer inositol was not greater than the passage of mannitol into brain when measured 15 s after a rapid injection of inositol and mannitol into the left common carotid artery. From the CSF, the clearance of inositol relative to inulin was saturable after the intraventricular injection of various concentrations of inositol and inulin. Moreover, a portion of the inositol cleared from the CSF entered brain by a saturable mechanism. In vitro, choroid plexuses, isolated from rabbits and incubated in artificial CSF, accumulated [3-H-labeled myo-inositol against a concentration gradient by a specific, active, saturable process with a K-t of 0.2 mM inositol. These results were interpreted as showing that the entry of inositol into the central nervous system from blood is regulated by a saturable transport system, and that the locus of this system may be, in part, in the choroid plexus.