The effects of phenobarbital (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) on liver hyperplasia, induction of microsomal enzyme activities, and two-stage hepatocarcinogenesis were evaluated in B6C3F1 female mice. For 4 weeks four groups of mice received PB (500 p.p.m. in the drinking water), TCPOBOP (3 mg/kg i.p. once every week), PB together with TCPOBOP or corn oil vehicle i.p. TCPOBOP induced liver hyperplasia and hypertrophy and increased p-nitroanisole-O-demethylase and aminopyrine-N-demethylase more than PB. Neither chemical changed UDPG-transferase activity. The association of PB and TCPOBOP gave the same effects as TCPOBOP alone. Other four groups of mice were treated with N-nitroso-N-diethylamine at 7 days of age and then, starting from 8 weeks of age, received the above specified weekly treatments for 20 weeks and were then sacrificed. Hepatocellular nodules > 150 μm were found in all animals of all groups. Due to increased size of the liver compared to controls, the number of nodules/cm ³ decreased after PB and TCPOBOP treatments given alone or together; however the mean volume of nodules and the percentage of liver volume occupied by nodules increased after TCPOBOP but not after BP treatment, and the association of PB and TCPOBOP was even more effective than TCPOBOP alone. Hepatocellular adenomas >2.4 mm in diameter were observed in 5 of 10 TCPOBOP-treated mice (total of 11 nodules) and in 5 of 11 mice that received PB plus TCPOBOP (total of 15 nodules). Hepatocellular carcinomas were seen in one mouse treated with PB and in three mice given PB and TCPOBOP.