continuous flux of the oxysterol 27-hydroxycholesterol (27-OHC) from the circulation across the blood-brainbarrier (BBB) into the brain. The major metabolite of 27-OHC in the brain is 7α-hydroxy-3-oxo-4-cholestenoic acid (7-HOCA). We confirm a recent report describing the presence of this metabolite in cerebrospinalfluid (CSF) at a relatively high concentration. A simple and accurate method was developed for