NKCC1 is highly expressed in the apical membrane of choroid plexus epithelial cells (Wu et al. 1998). It is not known, however, whether the transporter mediates the influx (Wu et al. 1998) or efflux of ions (Keep et al. 1994). In this study we have investigated the possible role of NKCC1 in the regulation of cell volume. The choroid plexus was dissected post mortem from the fourth ventricle of mice. Epithelial cells were isolated using dispase and cell volume measured by a video-imaging method. Wu et al. (1998) concluded that NKCC1 mediates ion influx based largely on the observation that 100 μM bumetanide caused a decrease in cell volume in isotonic solutions. A similar effect of 100 μM bumetanide was observed in 3 cells in this study; however, 10 μM bumetanide (a dose sufficient to inhibit NKCC1) had no effect on cell volume in isotonic solutions (n=7). The effect of 100 μM bumetanide on volume may therefore be due to non-specific effects of the drug. When exposed to hypertonic solutions cells immediately shrank as expected. In HEPES-buffered solutions a volume recovery was not observed (n=4). By contrast in HCO₃^–-buffered solutions a substantial regulatory volume increase (RVI) was observed (n=7). Neither 10 μM (n=5) nor 100 μM bumetanide inhibited this RVI (n=5). The RVI, however, was almost completely abolished four experiments by 10 μM methyl-isobutyl amiloride (an inhibitor of NHE; P< 0.05 by unpaired t test compared to control). These data indicate that NHE (probably coupled to AE) and not NKCC1 are responsible for the RVI in choroid plexus epithelial cells. In conclusion, NKCC1 does not appear to have a role in maintaining the volume of choroid plexus epithelial cells. The contribution of NKCC1 to ion fluxes in choroid plexus cells therefore remains to be established.