In a previous study, p-dichlorobenzene (pDCB), which is associated with tumorigenicity in male rat kidney and livers of mice of both genders, was found to produce acute increases in cell proliferation in those tissues. To determine whether sustained cell proliferation in the liver in susceptible species correlated with reported carcinogenic effects, we examined the effect of pDCB on cell proliferation in the livers and toxicity to the glutamine synthetase-expressing hepatocyte (GS+) subpopulation of male B6C3F1C3F1 mice and F344 rats. Mice were exposed for up to 4 weeks to 600, the maximally tolerated dose which increased liver tumors, 300 or 150 mg/kg. Rats were exposed to 300, 150 or 75 mg/kg for up to 4 weeks. In mice, the cumulative replicating fraction (CRF) in the livers of the high dose animals was significantly increased 16-fold at 1 week and 4-fold at 4 weeks. The CRF was also increased at 300 mg/kg at 1 week, but this subsided at 4 weeks. No increase was seen in the low dose group. In rats, the CRFs of the livers at 1 week were increased at 300 and 150 mg/kg, but returned to normal at 4 weeks. The size of the hepatic GS+ area was not affected in mice or in rats after 1 week of exposure, but comparable decreases were observed at all exposures at 4 weeks in mice. The data therefore suggest that sustained increases of cell division in the mouse liver may contribute to the increases in liver tumors. The transient increase in rat liver suggests that this is not sufficient to enhance tumor development. The absence of sustained increases of the CRFs at the low dose in mice, which was one-fourth of the hepatocarcinogenic dose, implies the existence of a threshold in pDCB hepatocarcinogenesis.