We described 7.49 tumors from 6 sources of rats raised at 7 laboratories. The sources were “Sprague-Dawley” from 4 commercial suppliers, Osborne-Mendel, and Oregon. Highly significant differences in the incidences of endocrine and mammary tumors were found. There were significant variations in the incidences of adrenal medullary tumors among rats from the same source raised at 2 laboratories. Of the rats with endocrine tumors, 9–15% had tumors in 2 or more endocrine glands. All but 1 of the testicular tumors occurred in Oregon rats. Thyroid and pituitary tumors were more common in females; adrenal medullary and islet cell tumors of the pancreas were more, frequent in males. Brain tumors occurred more often and at a younger age and were more apt to cause symptoms in males than in females. The incidence of tumors in “Sprague-Dawley” rats from different commercial sources varied as much from each other as from the other “strains.” Factors that can affect the incidence of tumors and the difficulties in comparing the rates of incidence reported in the literature were discussed. We stressed the need for extreme caution in evaluation of carcinogenicity studies conducted at different laboratories and/or on rats from different sources.