Factors affecting the distribution of iodide and bromide in the central nervous system

1. Even when a steady level of 131 I - is maintained in the blood for long periods, the uptake by brain and spinal cord is very small, and the possibility that this is due to an active transport of I - from brain-tissue to blood has been examined. 2. Most of the phenomena, however, can be explained on the basis of a slow passive diffusion across the blood—brain barrier associated with an active transport of 131 I - out of the c.s.f. across the choroid plexuses, so that, except possibly for the spinal cord, active transport from central nervous parenchyma into the blood need not be postulated. If it does occur, it contributes very little to the net exchanges between the three compartments, plasma, c.s.f. and extracellular fluid. 3. The steady-state distribution of bromide between plasma and c.s.f. is normally such that the concentration in the c.s.f. is only some 70% of that in plasma; it has been shown that this is most probably due to an active transport of Br - across the choroid plexuses.