The number of silver-stained nucleolar proteins (AgNOR) was counted in preneoplastic and neoplastic rat liver lesions induced by N-2-fluorenylacetamide (FAA) and was compared with that of bromodeoxyuridine (BrdU)-incorporating cells detected immunohistochemically using monoclonal antibody against BrdU. Male ACI/N rats were given diet containing 200 ppm FAA for 12, 16 or 20 weeks to induce hepatocellular foci and tumors. The mean numbers of AgNOR stained by a one-step silver colloid method and BrdU-labeling indices in various liver cell lesions were as follows: nontreated liver (n = 20), 1.20 and 0.08; nonlesional areas (n = 20), 1.33 and 0.13; altered liver cell foci (n = 80), 2.04 and 4.05 [eosinophilic cell type (n = 20), 1.78 and 1.82; clear cell type (n = 20), 1.45 and 1.77; basophilic cell type (n = 20), 1.99 and 4.58; hyperbasophilic cell type (n = 20), 2.94 and 8.02]; neoplastic nodules (n = 10), 3.11 and 2.99; hepatocellular carcinomas (n = 10), 7.22 and 8.29. Thus, the mean number of AgNOR and the value of BrdU-labeling index were well correlated and both values showed a stepwise increase from normal liver cells to liver cell carcinoma, although some scatter was present. These data suggest that mean number of AgNOR may reflect the cellular kinetics in rat hepatocarcinogenesis, and the one-step silver colloid method for demonstration of AgNOR may therefore be a simple and useful staining to examine the proliferative nature of cells.