Spontaneous Tumors in Sprague-Dawley Rats and Swiss Mice1

A spontaneous tumor incidence of 45% was noted in 360 Sprague-Dawley rats (179 males and 181 females) and a 26% incidence was seen in 254 Swiss mice (101 males and 153 females) used as untreated control animals in an 18month series of carcinogenesis experiments. The percentage of female rats with tumors was almost double that of males, which difference was accounted for chiefly by the high incidence of mammary tumors in the females. The largest number of rat tumors occurred in the endocrine system, mainly the pituitary and adrenal glands, with females exhibiting a higher incidence than males. There were no liver tumors. The largest group of mouse tumors occurred in the pul monary system, with a higher incidence in females than in males. Urinary system tumors were observed in the males but not in the females. Tumors of the integument, repro ductive organs, and the reticuloendothelial and lymphatic organs were also observed.