BACKGROUND. Imbalance between cell proliferation and cell apoptosis has been considered a key factor in carcinogenesis. Prostatic intraepithelial neoplasia (PIN) is the most likely precancereous lesion and represents the major target for chemoprevention of prostate cancer. The proliferative and apoptotic activities involved in the development of PIN remain to be elucidated. METHODS. Ventral prostates were removed from Noble rats that were treated with a combination of testosterone (T) and estradiol (E 2 ) for certain periods of time, and processed for histopathological grading. To evaluate the relationship between cell proliferation and apoptosis, immunohistochemistry for proliferating cell nuclear antigen (PCNA), Ki-67, and in situ DNA nick labeling (TUNEL) for identifying apoptotic cells, were performed on paraffin sections from prostate samples with PIN lesions. The results were correlated with expression patterns of Bcl-2 and Bax, two proteins related to cell survival and cell apoptosis. RESULTS. Pathologically, low-grade PIN (LGPIN) and high-grade PIN (HGPIN) were observed in ducts or alveoli after 3 and 5 months of T + E 2 treatment, respectively. Quantitative evaluation of Ki-67 showed an increased proliferative activity in HGPIN. In contrast to normal prostatic ducts and alveoli, which showed no positive staining for Ki-67 in the nuclei of luminal cells, 25% Ki-67-positive cells were detected in luminal cells of HGPIN. Only 7.5% Ki-67-positive cells were found belonging to the basal cell type. The Ki-67 index showed a higher growth rate from normal to HGPIN. The PCNA results showed a similar expression pattern to that of Ki-67 in normal prostate, LGPIN, and HGPIN. Apoptotic index (number of apoptotic cells/total number of cell counted) was significantly higher (P = 0.028) in HGPIN (3.23%) than in control prostate (1.19%). In contrast to control prostate, which showed no definite expression of Bcl-2, an intense positive expression of Bcl-2 in HGPIN was observed. Positive expression of Bax protein was observed in glandular epithelial cells of normal control prostate and HGPIN as well. CONCLUSIONS. Overexpression of Bcl-2 and higher Ki-67 or PCNA indices in HGPIN suggest that abnormal growth of premalignant lesions might result from an increase in cell proliferation. An increased apoptotic rate in HGPIN further implicates that active apoptosis may accelerate cell turnover in the development of premalignant lesions of the prostate.