Aquaporins, a family of water channels, provide a route for osmotically driven water passage across the plasma membranes of many cell types (Verkman & Mitra, 2000). The water channel aquaporin-5 (AQP-5) has been identified in mouse rat, and human sweat glands and postulated to be involved in sweat secretion (Nejsum et al., 2002). Horses and humans, unlike mice and rats, regulate their body temperature by the evaporation of sweat from the skin surface, however there is little information in the horse as to how sweat is formed. There is no information on the presence or absence of aquaporins in equine sweat glands, so the presence of AQP-5 was investigated.Skin biopsies were obtained from the neck region of horses that had been humanely killed for veterinary reasons (n=6) in accordance with current UK legislation. Samples were fixed, processed and wax embedded using standard techniques. Immunoperoxidase labelling was performed using a rabbit antibody raised against AQP-5 (DPC Biermann, Germany) and viewed by light microscopy. Protein samples extracted from unfixed, freshly isolated sweat glands and from an immortal cell line derived from equine sweat glands (E92, Wilson et al., 1993) were used for Western blot analyses.AQP-5-like immunoreactivity was localised only to the apical membrane of the luminal cells throughout the different regions of the sweat glands. Rat paw sweat gland positive controls elicited dark immunoreactivity restricted to the luminal membranes of the sweat gland cells. Pre-absorption of the antibody with control peptide abolished all specific staining in both the rat paw and equine sweat glands. Western blot analyses of freshly isolated glands and the equine cell line demonstrated the presence of a 28kDa band corresponding to the molecular weight of AQP-5 protein. The demonstration of AQP-5 in the equine cell line Western blots, suggests that repeated tissue culturing has not caused the cells to lose the ability to express AQP-5.Our study demonstrates that equine sweat glands like those of the human, mouse and rat, express the water channel AQP-5. Although, AQP-5 has been shown not to be essential for sweat secretion in paw glands in AQP-5-knockout mice (Song et al., 2002), mice, unlike the horse and human, do not use sweating for thermoregulation. It is therefore possible that AQP-5 may play a more functional role in thermoregulatory sweat secretion. Horses in hot humid climates can develop a condition known as dry coat whereby the animals progressively fail to produce sweat secretions. The cause of this condition is not known and we are currently investigating whether AQP-5 expression is altered in anhidrotic horses.