In a two-year (106 week) feeding study the chronic toxicity and carcinogenicity of bis(tri-n-buty1tin) oxide (TBTD) were investigated in the rat. Groups of 60 weanling Histar rats of each sex were fed 0, 0.5, 5 or 50 mg TBTO/kg diet. Mortality, general observations, body weight, feed and water consumption were recorded throughout the study. In 10 rats per sex and group clinical chemistry, urinalysis, and hematology were performed after 3, 12 and 24 months, whereas endecrinology studies, organ weight analyses, total tin determinations of liver and kidney, gross and histopathology were conducted after 12 and 24 months. All animals in the study were monitored for the occurrence of tumors. Mortality was increased at a dietary level of 50 ag/kg in both sexes towards the end of the study. Common clinical signs in moribund rats during the second year of the experiment consisted of emaciation, ataxia and depression. The feed consumption of the male animals at 0.5 ag/kg was increased between weeks 24 and 80. Feed and water consumption were increased in the malsa fed 5 ag/kg throughout the study and in those of the 50 ag/kg group after week 24. From week 67 onward, body weight decreased in the male animals of the high dose group, while the females of that group showed no more weight gain.