In lifespan studies, outbred male and female Wistar rats were given either technical DDT mixed into the diet at a dose of 500 parts per million (ppm) or phenobarbital-sodium dissolved in drinking water at a dose of 500 ppm. Liver-cell tumors developed in treated animals but not in controls. The incidence of liver tumors was 45% in the DDT-treated group and 44% in the phenobarbital-sodium-group. When evaluated by sex, DDT-treated females and males had incidences of 56% and 35%, respectively, while in the phenobarbital-sodium group, the respective incidences were 32% and 59% in females and males. These data show a varying susceptibility between the sexes, with regard to induction of liver-cell tumors by the two compounds. In both treated groups, the number of nodular tumors per rat and the average size increased with age and were greater in females. None of these tumors and metastasized. Histologically, the liver tumors were nodular growths, which compressed surrounding parenchyma did not infiltrate it. The total incidence of extrahepatic tumors was higher in controls than in treated animals. In this connection, we must mention the apparent, but not significant, reduction of adrenal tumors in treated rats compared to the controls.