Four groups of 60 male Alpk:APfSD rats were fed different nutritionally comparable diets for 107 weeks with interim sacrifices at 26, 53 and 77 weeks in order to compare the effect of diet on the incidence of oral disease. Changes in the oral and nasal cavities were assessed by histopathology. Oro-nasal fistulation and severe periodontitis were associated with diets containing fibres originating from oats and barley. An expanded ground diet induced the most severe lesions. Aspects of the pathogenesis of the lesions observed are discussed.