The DNA content of cell nuclei of liver lesions induced in inbred male ACI rats by administration of 0.02% N-2-fluorenylacetamide (2-FAA) diet was measured with a microspectrophotometer in Feulgen-stained specimens. Altered liver foci of the eosinophilic, clear, and simple basophilic (toluidine blue weakly positive or negative) types had modal DNA values with a wider deviation (2C-10C) than the euploid pattern of normal hepatocytes (2C-6C). However, the DNA histograms of cells in hyperbasophilic (intensely toluidine blue-positive) foci were aneuploid (2C-18C) and resembled those of cells in hepatocellular carcinomas (2C-23C). Conversely, the modal DNA values of neoplastic nodules showed a narrower range (2C-12C). These results confirm that the hyperbasophilic foci may be a population similar to carcinoma in situ or may be a direct precursor lesion of hepatocellular carcinoma. In the livers from 4 humans, 3 of whom had a history of long-term use of some drugs, multiple hyperplastic foci resembling eosinophilic or clear cell foci in rats were identified. The modal DNA values of the cell nuclei in these foci showed a wider deviation (2C-10C) than in surrounding hepatocytes (2C-6C). In 2 patients, the lesions contained neoplasms and, therefore, hyperplastic foci in humans may be related to tumor development.