Background: Bisoprolol and metoprolol are moderately li-pophilic, beta(1)-selective betablockers reported to cause adverse effects in the central nervous system (CNS), such as sleep disturbance, suggesting that both drugs may reach rel-evant concentrations in the brain. CNS beta(2)-receptor blockade has been suspected to be related to such effects. The higher molecular size of bisoprolol (325 Dalton) and the higher beta(1)-selectivity compared to metoprolol (267 Dal-ton) would suggest a lower rate of CNS effects. Methods: To address the pharmacokinetic background of this assump-tion, we quantified to which extent these beta(1)-blockers are able to enter the cerebrospinal fluid (CSF) in 9 (bisoprolol group) and 10 (metoprolol group) neurological patients who had received one of the drugs orally for therapeutic purpos-es prior to lumbar puncture. We quantified their total con-centrations by liquid chromatography/tandem mass spec-trometry in paired serum and CSF samples. Results: Median (interquartile range) in CSF reached 55% (47–64%) of total serum concentrations for bisoprolol and 43% (27–81%) for metoprolol, corresponding to 78% (67–92%) and 48% (30–91%) of respective unbound serum concentrations. Conclusion: The extent of penetration of bisoprolol and metoprolol into the CSF is similar and compatible with the assumption that both drugs may exert direct effects in the CNS.