Hepatocarcinogenesis was examined in an initiation--promotion protocol with a single initiating dose of 7,12-dimethylbenz[a]anthracene (DMBA) followed by promotion with phenobarbital (PB) in the Nagase analbuminemic rat (NA), the Sprague-Dawley rat (SD) and their F1 crosses. All rats received a 70% partial hepatectomy, followed at 24 h by 30 mg DMBA/kg body wt or the solvent. After a 2 week recovery following surgery, half of the solvent control and initiated groups received either basal diet or promotion with 0.05% PB mixed into the basal NIH-07 diet. After 12 weeks of promotion, the rats were killed and the livers perfused and fixed in situ with paraformaldehyde. Liver slices were paraffin embedded and stained for the placental isozyme of glutathione S-transferase (PGST). The number of altered hepatic foci (AHF) expressing PGST per liver was determined by quantitative stereology and used as an endpoint for comparison of initiation in the rat strains. The NA rat had a lower response to this initiation-promotion protocol than did the SD rat. The F1 progeny of the male NA and female SD rats were more similar to the NA parent in their responsiveness to initiation, whereas the F1 progeny of the female NA and male SD were similar to the SD parent in this respect. Putative mutagenesis and carcinogenesis were examined in the F1 progeny of the female NA and male SD rat. In these rats, serial liver sections were stained either for albumin to detect putative mutations at that locus, or PGST to identify putatively initiated hepatocytes. In the NA/SD F1, the number of single hepatocytes with a putative mutation at the albumin locus was the same (3.7 x 10(5)/liver) as those expressing a common marker of preneoplasia (PGST). The number of AHF expressing PGST was approximately 5% that of the single cells exhibiting an altered expression of albumin or PGST, indicating a possible quantitative correlation between initiation and mutation in vivo when individual hepatocytes with altered gene expression were counted. These studies also suggest that only a subpopulation of the putatively initiated hepatocytes expands clonally in the presence of the promoting agent, PB. The progeny of the female NA rat crossed with the SD male rat appears to provide a useful model in which to compare mutation and carcinogenesis simultaneously in vivo.