The study of cell proliferation is a useful tool in the fi elds of toxicology, pathophysiology and pharmacology. Cell proliferation and its degree can be evaluated using 5-bromo-2'- deoxyuridine which is incorporated into the newly synthesized DNA. The aim of this study was the optimization of subcutaneous application of 5-bromo-2'-deoxyuridine implantation for continuous and persistent marking of proliferating cells in the rat forestomach. 3-tert-Butyl-4-hydroxyanisole was used as the agent that ensures cell proliferation. In order to determine the optimal dose for proliferating cells labeling, 5-bromo-2'-deoxyuridine doses of 50 mg, 100 mg, 200 mg or 350 mg were implemented 2 days prior to sacrifi ce by fl at-faced cylindrical matrices. Immunohistochemical analysis using 5-bromo-2'-deoxyuridine in situ detection kit was performed for the detection of 5-bromo-2'-deoxyuridine labeled cells. The results showed that for adult rats, the optimum 5-bromo-2'-deoxyuridine dose is 200 mg per animal for subcutaneous application. The here described manner of 5-bromo-2'-deoxyuridine in vivo labeling provides a simple, effi cient, and reliable method for cell labeling, and at the same minimizes stress to animals. [ABSTRACT FROM AUTHOR]