5 groups of 14-day ICR male mice received intraperitoneally diethylnitrosamine in the dose 50 mcg/g body weight. After weaning, they were kept in standard laboratory conditions. When aged 6 months, the mice of the 1st group were sacrificed and the number of iron-negative premalignant nodules in their livers as well as adenomas in the lungs were recordered. Since then, mice of the 3rd and 5th groups received 0.0002% L-thyroxine as drinking water throughout the experiment. Control mice of the 2nd and 4th groups were given tap water. At 9 months of age, total number of hepatic nodules in mice of group 2 was slightly (40%) higher whereas the number of gross nodules (2-5 mm in diameter) and extremely gross ones (> 5 mm) were 3- and 6-fold higher than in mice of group 1. After additional 2 months, the number of tumor nodules in the liver of control mice of group 5 could not be counted because of their mixed growth. By contrast, in thyroxine-treated mice, the number of gross nodules, as well as total number of tumor nodules in the livers were not higher than those in control mice of group 1. The data obtained indicate that exogeneous thyroxine inhibits growth of liver tumors even at the late stages of carcinogenic process. In addition, it increases the food intake and relative weight of kidneys and decreases relative weight of testes but does affect growth of pulmonary adenomas in male mice under study.