title: Bioassay of N,N'-diethylthiourea for possible carcinogenicity.
abstract: A bioassay for the possible carcinogenicity of N,N'-diethylthiourea was conducted using Fischer 344 rats and B6C3F1 mice. N,N'-Diethylthiourea was administered in the feed, at either of two concentrations, to groups of 50 males and 50 females of each species. Twenty animals of each sex and species, except for 19 male mice, were placed on test as controls. The high and low dietary concentrations of N,N'-diethylthiourea were, respectively, 250 and 125 ppm for rats and 500 and 250 ppm for mice. The compound was administered in the diet for 103 weeks, followed by an observation period of 1 week for all dosed groups. There were no significant positive associations between the dosages of N,N'-diethylthiourea administered and mortality in rats or mice of either sex. Adequate numbers of animals in all dose groups survived sufficiently long to be at risk from late-developing tumors. Compound-related mean body weight depression was apparent among dosed male and female mice when compared to their respective controls,indicating that the concentrations of N,N'-diethylthiourea administered to mice may have approximated the maximum tolerated dosages. There were statistically significant elevated incidences of follicular-cell carcinomas of the thyroid in high dose male rats. In addition, there were statistically significant elevated incidences of a combination of thyroid follicular-cell carcinomas and follicular-cell adenomas in high dose male and female rats. Under the conditions of this bioassay, N,N'-diethylthiourea was carcinogenic to Fischer 344 rats, causing follicular-cell carcinomas of the thyroid in males and follicular-cell neoplasms of the thyroid in females. There was no evidence for the carcinogenicity of the compound in B6C3F1 mice.