Effect of subsequent treatment of chloroform or phenobarbital on the incidence of liver and lung tumors initiated by ethylnitrosourea in 15 day old mice.

The effect of subsequent administration of chloroform or phenobarbital on the incidence of ethylnitrosourea (ENU) initiated liver and lung tumors was investigated. Fifteen day old Swiss mice were administered ENU, and at weaning they started to receive either 1800 p.p.m. chloroform or 500 p.p.m. sodium phenobarbital in their drinking water. The mice continued to receive either chloroform or phenobarbital until 51 weeks of age. They were sacrificed at 52 weeks of age. ENU at 5 and 20 mg/kg, caused a dose-dependent increase in liver and lung tumors. The male mice were more sensitive to the induction of liver tumors, while no sex preference was observed for the induction of lung tumors. In male mice chloroform inhibited, while in female and male mice phenobarbital promoted spontaneous and ENU-induced liver tumors. Subsequent treatment with either chloroform or phenobarbital did not affect the incidence of ENU-induced lung tumors. In conclusion, when administered in the drinking water, chloroform inhibited while phenobarbital promoted hepatocarcinogenesis in mice.