Unbound fractions in mouse brain and plasma were determined for 31 structurally diverse central nervous system (CNS) drugs and two active metabolites. Three comparisons were made between in vitro binding and in vivo exposure data, namely: 1) mouse brain-to-plasma exposure versus unbound plasma-to-unbound brain fraction ratio (fu(plasma)/fu(brain)), 2) cerebrospinal fluid-to-brain exposure versus unbound brain fraction (fu(brain)), and 3) cerebrospinal fluid-to-plasma exposure versus unbound plasma fraction (fu(plasma)). Unbound fraction data were within 3-fold of in vivo exposure ratios for the majority of the drugs examined (i.e., 22 of 33), indicating a predominately free equilibrium across the blood-brain and blood-CSF barriers. Some degree of distributional impairment at either the blood-CSF or the blood-brain barrier was indicated for 8 of the 11 remaining drugs (i.e., carbamazepine, midazolam, phenytoin, sulpiride, thiopental, risperidone, 9-hydroxyrisperidone, and zolpidem). In several cases, the indicated distributional impairment is consistent with other independent literature reports for these drugs. Through the use of this approach, it appears that most CNS-active agents freely equilibrate across the blood-brain and blood-CSF barriers such that unbound drug concentrations in brain approximate those in the plasma. However, these results also support the intuitive concept that distributional impairment does not necessarily preclude CNS activity.