It is well documented that the incidence of renal disease, and therefore renal dysfunction, increases with age in many species of mammals. Such alterations in renal structure and function may significantly affect long-term toxicology studies. The purpose of this study was to assess the temporal evolution of glomerulosclerosis, an important renal lesion, in laboratory housed dogs, an important model system in chronic toxicological studies. We histopathologically examined representative sections of dog kidneys, quantified glomerular lesions (using the 0-5 scale of the World Health Organization classification system) and performed of statistical analysis of the extent and distribution of such changes. The kidney samples were obtained by necropsy, and occasionally biopsy, procedures from a collection of 159 purebred Beagle dogs maintained for their entire lifespan in well-controlled conditions. The lesions were correlated with sex, age, and intra-renal location of affected glomeruli to determine the relationship of each in the development of glomerulosclerosis. All dogs examined had some degree of glomerulosclerosis. In the youngest (up to 2 years of age), this was minimal, but was more advanced by middle age (3-7 years). The condition progressed with further aging and was associated with progressive fibrosis and tubular loss. Location and advancing age were significantly related to the development of glomerulosclerosis such that as age increases, the incidence of glomerulosclerosis increases, with the inner medullary ray and inner cortex demonstrating the highest occurrence. Using immunohistochemical analysis, the percentage of type IV collagen within glomeruli was determined. No significant increase in type IV collagen in glomeruli due to age or location was seen. An increase in type III or type V collagen within glomeruli was not apparent either, upon visual examination. This study indicates that renal lesions, including glomerulosclerosis, occur commonly and progress over the lifetime in a genetically similar population of laboratory Beagle dogs maintained under optimal standard environmental conditions. Such typical, age-related change needs to be taken into consideration when conducting chronic toxicological experiments using such animals. / Master of Science