Membrane transporters expressed in the choroid plexus (CP) are involved in the transport of substances between the blood and cerebrospinal fluid (CSF). Carnitine/organic cation transporter 1 (OCTN1, also known as SLC22A4) is expressed in rodent CP; however, its specific roles in blood-CSF transport remain unclear. Therefore, in this study, we aimed to evaluate the potential role of OCTN1 in the elimination of substances from CSF. Tritium-labeled ergothioneine ([ 3 H]ERGO), a typical in vivo substrate of OCTN1, was injected into the lateral ventricles of wild-type and octn1 gene knockout (octn1 -/- ) mice. Clearance of [ 3 H]ERGO from CSF was higher than that of the bulk flow marker, [ 14 C]mannitol, in wild-type mice. However, [ 3 H]ERGO clearance was significantly lower in octn1 -/- mice than in wild-type mice. Furthermore, OCTN1 expression in CP was determined via immunohistochemical analysis. CP/CSF ratio of [ 3 H]ERGO was significantly lower in octn1 -/- mice than in wild-type mice. These results suggest that OCTN1 is functionally expressed in CP and involved in the elimination of ERGO from CSF in mice.