To investigate the mechanisms of [Cl^-]_i accumulation in immature cortical neurons gramicidin-perforated patch-clamp recordings and MEQ-microfluorimetric measurements were per-formed on identified Cajal-Retzius (CR) cells in tangential slices of neonatal (P0-3) rat brains. Perforated patch-clamp recordings revealed an GABA-reversal potential (E_GABA ) of -39.3 ± 3.1 mV (n=9) in CR cells corresponding to a [Cl^-]_i of about 24 mM. Application of 50 mM bumetanide and Na ⁺-free extracellular solution reduced E_GABA significantly to -46.7 ± 1.9 mV (n=7) and -50.2 ± 2.1 mV (n=6) and affected the kinetics of [Cl^-]_i regulation, indicating a bumetanide sensitive and Na⁺ dependent regulatory mechanism. Similar results were obtained by measuring [Cl^-]_i with the Cl^-sensitive dye MEQ. In-situ hybridisation showed a strong NKCC-1 signal in the marginal zone of the developing neocortex and single cell RT-PCR confirmed the expression of NKCC-1, but also of KCC-2 and ClC-2 in CR cells. These results suggest that CR cells have a high [Cl^-]_i, maintained by the activity of the bumetanide-sensitive Na⁺, K⁺- 2Cl^--cotransporter NKCC1. Supported by DFG-grants Ki 835/2 to WK, Lu 375/4 to HJL and JSPS Grant #13210065 to AF