The question addressed was whether stimulation of cell proliferation could be responsible for tumor induction in the forestomach by styrene 7,8-oxide (SO). Male F344 rats were treated for 4 weeks with 0, 137, 275, and 550 mg/kg SO by p.o. gavage 3 times/week. Positive controls received 0, 0.5, 1, and 2% butylated hydroxyanisole (BHA) in the diet for 4 weeks. Twenty-four h before termination of the experiment, the rats were implanted s.c. with an osmotic minipump delivering 5-bromo-2'-deoxyuridine (BrdU). Cell proliferation in the forestomach was assessed by immunohistochemistry for BrdU incorporated into DNA. Cell number/mm section length and fraction of replicating cells (labeling index) were determined in 3 domains of the forestomach, the saccus caecus, the midregion, and the prefundic region. With the exception of the prefundic region of the low-dose SO group, a significant increase of the labeling index was found in all regions both with SO and BHA. Rats treated with BHA showed, in addition, a dose-dependent increase in number and size of hyperplastic lesions. This was most pronounced in the prefundic region where carcinomas were reported to be localized. In this region, the number of dividing cells/mm section length was increased up to 17-fold. With SO, only marginal morphological changes were occasionally observed, despite the fact that the respective long-term treatment had been reported to result in a higher carcinoma incidence than treatment with BHA. It is concluded that the rate of replicating cells alone, numerically expressed by the labeling index, is an insufficient tool for interpreting the role of cell division in carcinogenesis. It is postulated that SO and BHA induce forestomach tumors via different mechanisms. While hyperplasia in the prefundic region most likely dominates the carcinogenicity of BHA, a mechanism combining marginal genotoxicity with strong promotion by increased cell proliferation appears to be involved in the tumorigenic action of SO.