BACKGROUND 1,1 ,2,2-Tetrachloroethane was widely used in the production of solvents and pesticides. Its production ended in the 1990s, but it is a major component of waste sites. We studied the effects of 1,1 ,2,2-tetrachloroethane on male and female rats and mice to identify potential toxic hazards to humans. METHODS Because 1,1,2,2-tetrachloroethane can evaporate easily, we enclosed it in starch microcapsules and placed them in the feed of rats and mice for 14 weeks. Male and female rats received up to 4,600 parts per million (ppm) 1,1 ,2,2-tetrachloroethane (equivalent to 0.46%) and mice received up to 9,100 ppm (0.91%). Control animals received empty starch microcapsules in their feed. Tissues from more than 40 sites were examined in all control and high-dose animals; tissues with lesions were examined in the lower exposure groups until no lesions were observed. RESULTS Rats receiving 1,180 ppm or more 1,1,2,2-tetrachloroethane and mice receiving 2,300 ppm or more weighed less than the control animals. Male and female rats given 1,1 ,2,2-tetrachloroethane had pale and diseased livers and also had atrophy of the bone marrow and of the genital systems. Male and female mice given 1,1,2,2-tetrachloroethane had lesions of the liver and the bile duct. CONCLUSIONS We conclude that 1,1,2,2-tetrachloroethane at doses greater than 590 ppm in the feed was toxic to the liver of male and female rats. In mice, 1,1 ,2,2-tetrachloroethane was already known to cause cancer after long-term exposure. In these 14-week studies, 1,1 ,2,2-tetrachloroethane was toxic to the livers of male and female mice.