Gamma-glutamyltranspeptidase-positive hepatocyte foci were produced in female rats given a single dose of diethylnitrosamine neonatally after birth and, after weaning, a diet containing phenobarbitone for 30 wk. The nucleator method, a new stereological approach, provided an efficient, unbiased estimate of mean cell volume in focal lesions and extrafocal areas. It also provided an unbiased sample of cells to estimate hepatocyte nuclear volume and the percentage of binucleated cells. The results showed an increase in the mean volume of mononucleated cells--from 4,700 micron3 in extrafocal areas to 12,700 micron2 in foci--and of binucleated cells--from 6,900 micron3 to 25,000 micron3. This demonstrated the hypertrophic effect of the carcinogenic treatment in focal lesions. A striking reduction in the proportion of binucleated cells was also observed in the preneoplastic lesions. Nuclear volume measurements from mononucleated and binucleated hepatocytes were used to assess ploidy. An apparent increase in nuclear ploidy, with no change in cellular ploidy, was noted in focal tissue when compared with nonfocal tissue. This appeared to be caused by an increase in mononucleated tetraploid cells and a reduction in binucleated cells with two diploid nuclei, indicating an altered mitotic mechanism in focal lesions. The significance of these changes in cell volume, apparent ploidy levels and binuclearity in preneoplastic foci is discussed in relation to the hepatocarcinogenic process.