Kinetics of water movements linked to transmembrane ion movements may contribute to activity-dependent changes in excitability of unmyelinated nerve fibers. Recently, immunohistochemical data and functional experiments indicated that peripheral sensory neurons express aquaporin-1 (AQP1), a water-permeable membrane protein (Oshio et al. BBRC 314; 2006, 1022). However, in another publication, the expression of AQP1 was attributed to non-myelinating Schwann cells (Gao et al. Glia 53; 2006, 783). In the present study, we have looked for AQP1-positive nerve fibers in transgenic mice which express the enhanced green fluorescent protein (eGFP) under the control of the human glial fibrillary acidic protein (GFAP) promoter (Nolte et al. Glia 33; 2001, 72). Using confocal microscopy, single non- myelinating Schwann cells can be identified in dorsal roots, sural nerve, vagus nerve, and spinal ganglia. A selective co-localisation of AQP1-positive nerve fibers with non-myelinating Schwann cells was observed in all preparations. However, a separation of the axonal and the Schwann cell membrane was not possible. On the other hand, in dorsal root ganglia, only cell bodies of small sensory neurons were immunopositive for AQP1 (Oshio et al. BBRC 314; 2006, 1022). This finding indicates that AQP1 is expressed in the membrane of unmyelinated axons rather than in non-myelinating Schwann cells.