Neoplastic and nonneoplastic lesions in 975 male and 970 female Osborne-Mendel rats used as controls in carcinogenesis tests were tabulated and evaluated. Three types of controls were considered—untreated controls (245 males, 245 females), controls administered corn oil in feed (530 males, 525 females), and controls administered corn oil by gavage (200 males, 200 females). Few neoplasms were seen in rats less than 18 months of age; the incidence of tumors markedly increased between 18 and 24 months of age. The incidence of some of the more common neoplasms, in which the combined incidence was greater than 1%, varied among the control groups, and in most cases the differences in incidences were not statistically significant. Hemangiosarcomas at all sites and C-cell adenomas of the thyroid in female rats were marginally significantly lower in controls given corn oil by gavage than in the other two groups. The occurrence of adenocarcinomas of the mammary gland was significantly higher in male controls given corn oil by gavage than in the other two groups. In untreated controls, the incidences of adrenal cortical adenomas in males and females and of pheochromocytomas in males were significantly higher than in both groups given corn oil. In rats administered corn oil in the feed, the incidences of pituitary adenomas in males and females, follicular cell adenomas of the thyroid in males, and endometrial stromal polyps in females were significantly higher than the incidences in the other two groups. The possible reasons for these differences are discussed. However, the types of tumors seen in the three control groups were morphologically comparable. The tumors observed included two types that had not previously been well characterized in Osborne-Mendel rats—malignant fibrous histiocytomas and lipomatous tumors of the kidney. Incidences of the more common nonneoplastic lesions are reported.
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