Abstract
Pathology, microbiology, and selected serum chemistries were evaluated in 144 male Fischer rats from 4 to 33 mo of age. The rats were reared and maintained under barrier conditions, which successfully excluded the introduction of major infectious disease agents throughout the entire study, including Mycoplasma pulmonis. A wide variety of pathology was found and tabulated, and many lesions were found to increase in severity and incidence with age. There was a high correlation of renal disease severity with increasing age. Serum total protein and albumin decreased with age, while alpha-1 globulin and cholesterol increased.