The respiratory epithelial response of the adult horse was evaluated after being housed on pasture and in an enclosed stable for four week periods. six thoroughbred cross horses, ranging from 3 to 6 years old, were housed on a pasture for 2 months, and evaluated at the end of this period. After an additional month on pasture, horses were then moved to a barn and housed in stalls for four weeks. Samples were obtained at the end of this time and horses were returned to pasture for two months. Final samples were then collected. Each evaluation included a physical examination; cytological evaluation of transtracheal aspirate, and histological evaluation of tracheal epithelial biopsies obtained at the level of the main stem bifurcation. Endoscopic evaluation also permitted scoring of pharyngeal lymphoid hyperplasia, and tracheal mucous secretions. No significant changes were noted in any of the parameters evaluated in this study. These results suggest housing normal horses in a stable environment does not an elicit an immediate change in the volume or composition of epithelial secretions. Evaluation of the tracheal epithelium biopsies also did not reveal histologic changes or inflammatory infiltration in response to short-term housing in the stable environment. / Master of Science