Aquaporins, membrane water channels, provide a possible explanation for the route of osmotically driven water passage across the plasma membranes of many cell types. The aquaporin-5 (AQP-5) subtype has been localised in mouse, rat (Nejsum et al., 2002) and horse (Bovell et al., 2004) sweat glands. The human eccrine sweat gland differs from the other sweat glands by having both secretory and reabsorptive portions. The secretory coil has two major cell types defined as the dark and light cells, with the latter thought to have a major involvement in the production of sweat fluid. Although AQP-5 has been shown to be present in the reabsorptive duct portion (Nejsum et al., 2002), its presence in the secretory coil cells has not been completely elucidated. Using immunohistochemical techniques we have investigated the presence of AQP-5 within the secretory coil and reabsorptive duct of the human eccrine sweat gland. Skin biopsies were obtained with informed patient consent and with local medical ethical committee approval. Samples were fixed, processed and embedded in paraffin wax using standard techniques. Staining was performed using a rabbit anti-AQP-5 antibody (1:500, DPC Biermann, Germany) with an avidin-biotin procedure for immunohistochemistry and an FITC labelled secondary antibody for immunofluorescence. Human glands were found to have AQP-5-like immunoreactivity localised to the membranes of what appeared to be the light secretory cells of the eccrine secretory coil. The light cells but not the dark cells of the coil have been shown to express carbonic anhydrase II (CAII) (Brigmann et al., 1983) making CAII a good marker for light cells. Immunoreactivity showed that AQP-5 and CAII stained the same cells. Additionally, the reabsorptive duct was found to have AQP-5-like immunoreactivity in the apical membranes. The localisation of AQP-5-like immunoreactivity at the membranes of the light cells of the eccrine secretory coil, provides evidence that these channels offer a possible route of water transport and are consequently implicated in the production of sweat. The reabsorptive duct is thought to be relatively impermeable to water although restricted movement may occur at the apical membrane (Burry et al., 1997). The localisation of AQP-5 at the apical membrane of the reabsorptive duct provides an explanation for this limited water movement and confirms the previous work of Nejsum and colleagues (2002).