Incidence of neoplastic lesions in untreated Sprague-Dawley rats (1340 males and 1329 females) used as controls in 17 carcinogenicity studies are tabulated and evaluated. In male rats, the most common neoplasms were benign pheochromocytomas and keratoacanthomas (4.0% in each case) followed by pancreatic islet cell adenomas (3.7%), thyroid parafollicular cell adenomas (3.6%), fibromas and squamous cell papillomas of the skin and hepatocellular adenomas (2.0% in each), malignant lymphoma lymphocytic (1.9%), histiocytic sarcomas (1.4%), and adrenal cortical adenomas (1.2%). In female rats, the most common neoplasms were of mammary gland origin (31.3%: fibroadenoma 19.0%, adenocarcinomas 8.8%, and adenomas 3.5%) followed by thyroid parafollicular cell adenomas (2.9%), uterine endometrial stromal polyps (2.6%), adrenal cortical adenomas (1.9%), malignant lymphoma lymphocytic (1.6%), fibromas in the skin (1.3%), and pancreatic islet cell adenoma (1.1%). Metastases were observed from pheochromocytomas, hepatocellular carcinomas, nephroblastomas, renal pelvis transitional cell carcinoma, interstitial cell tumor and seminoma of the testes, Zymbal's gland adenocarcinomas, and mammary adenocarcinomas.