Microscopic observation data collected from approximately 1800 male and female Sprague Dawley (SD) control rats used on 104-week carcinogenicity studies performed at North American Labcorp Early Development, Inc, Madison, WI, were retrospectively evaluated for spontaneous nonneoplastic findings. This study provides incidence of the most common spontaneous nonneoplastic microscopic findings in each organ system of SD rats encountered during 104-week carcinogenicity studies. Some of the most common spontaneous background findings were cardiomyopathy
chronic progressive nephropathy
uterine cystic endometrial hyperplasia
prostate inflammation
pulmonary alveolar macrophage infiltrates
hepatocyte vacuolation, bile duct hyperplasia, and basophilic foci in the liver
pancreatic fibrosis
splenic extramedullary hematopoiesis and pigment
decreased lymphocytes and epithelial hyperplasia in the thymus
ventral brain compression
cystic degeneration and hyperplasia of the adrenal cortex
and mammary gland hyperplasia. The most common nonneoplastic findings in male SD rats were chronic progressive nephropathy (80.9%) and rodent progressive cardiomyopathy (73.2%). The most common nonnenoplastic findings in female SD rats were cystic degeneration of the adrenal cortex (64.7%) and ventral compression of the brain due to pituitary neoplasms (62.7%).