Little information exists on the concentrations of recent antidepressants and their metabolites in cerebrospinal fluid (CSF). Using a stereoselective method, we measured plasma and CSF levels of mirtazapine (MIR), N-demethylmirtazapine and 8-OH-MIR in 3 depressed patients treated with racemic MIR (45 mg/day) for 4 weeks. S-(+)-MIR is considered to be the antidepressant enantiomer, but only R-(-)-MIR reached measurable concentrations in CSF. For R-(-)-MIR, the CSF/plasma ratio varied between 0.08 and 0.31. Further studies are needed to test the hypothesis that there are possible differences in the transport mechanisms of the enantiomers of MIR at the blood-CSF barrier. 2006 S. Karger AG, Basel