Cell proliferation index in the thyroid is regarded as an important marker for predicting the prognosis and for differential diagnosis of thyroid tumors. Among the methods for the detection of cell proliferation in histological sections, immunohistochemistry for PCNA and BrdU are the most preferable ones. In this study, we evaluated the regeneration process of the thyroid in the rat after surgical resection with special respect to the cell proliferation revealed by both PCNA and BrdU immunohistochemistry, and compared these two results. Rat thyroid was resected bilaterally, about 50~60% of the total lobe in volume, and the tissues were obtained 1, 2, 3, 4, 5, and 7 days after operation, immunohistochemistry was done, and positive and negative cells were counted. The regeneration pattern reflected by cell proliferation was basically same in both PCNA and BrdU immunohistochemistry, except that the positive ratio for PCNA was 4~6 fold higher than that for BrdU in general. In the control thyroid, positive cells were rare. However, the ratio of positive cells increased immediately after the operation, on day 1, and reached at its peak on day 2. The high ratio sustained until day 4, after which the ratio declined abruptly. Through the experimental days, positive cells were more crowded in the area adjacent to the resection plane than elsewhere, suggesting that the areas are more active regenerating focus. Although the pattern of proliferation was same both in PCNA and BrdU immunostaining in terms of time and localization, BrdU staining was easier to read and seeded to reflect cell prolifer-ations more specifically than PCNA staining.