Disulfonic stilbenes (DIDS and SITS) were used to probe the nature of Cl transport in choroid plexus (CP) of adult rats. DIDS was more effective than SITS in reducing in vitro choroid cell [Cl]. Intravenously- or intraventricularly-administered DIDS substantially decreased 36Cl transport from blood to CP to CSF. Rate of CSF renewal was diminished nearly 30% by DIDS. These inhibitory findings implicate an anion antiporter for mediating Cl flux (and thus water) between blood and CSF.