We measured in simultaneously withdrawn cerebrospinal fluid (CSF) and serum samples from 56 endocrinologically grossly normal patients the concentrations of several lipophilic unconjugated steroids [i.e. dehydroepiandrosterone (DHEA), androstenedione, cortisol, progesterone, testosterone] and their hydrophilic counterparts, i.e. DHEA-sulfate, or hydrophilic binding proteins, i.e. albumin, sex hormone-binding globulin (SHBG) and corticosterone-binding globulin (CBG). CSF levels of total (i.e. free plus protein-bound) DHEA, androstenedione, cortisol, progesterone, and testosterone were found to be in the 0.02-2 nM range and only cortisol reached levels approximately 20 nM. These values were of the same order of magnitude as the reported and calculated free serum levels of these steroids. In patients with disturbed (abnormally leaky) blood-CSF barrier (BCB) function, CSF levels of these steroids were not different from those with intact BCB, in contrast to DHEA-sulfate, CBG and SHBG whose CSF levels were significantly elevated, that is similar (i.e. 2-5) fold as those of albumin. In vitro demonstrated low affinity (micromolar) interactions of steroids with neuronal membrane-bound neurotransmitter receptors should be considered in perspective to the here reported finding that steroids occur in vivo at best in nanomolar concentrations in the CSF. Whether in other extracellular fluid compartments of the brain higher levels of steroids than in CSF can accumulate is as yet not clear. Very probably, pathological production or excessive dosage of steroids that are negligibly bound to SHBG or CBG will produce CSF and brain levels in the near micromolar range.