The time course of 5-bromo-2’-deoxyuridine (BrdU) uptake by medial and neointimal smooth muscle cells was examined in rat carotid arteries at periods of 1-20 days after balloon catheter injury. DNA-incorporated BrdU was determined by the indirect immunoperoxidase technique in both plastic- and paraffin-embedded specimens. The BrdU labeling index (LI) peaked at 48 h in the media (19.9% paraffin, 9.4% plastic) and at 5 days in the neointima (55.4% paraffin, 60.2% plastic). Immunohistochemistry for the endogenous marker proliferating cell nuclear antigen was variable and generally less reliable than BrdU. Using total viable medial smooth muscle cell number as an index of medial injury, the LI for BrdU was directly proportional to the extent of injury at 48 h, and correlated with loss of actin immunoreactivity. BrdU immunohistochemistry is a reliable alternative to [<sup>3</sup>H]-thymidine autoradiography for short-term labeling in this model. Proliferation of smooth muscle cells in this model is likely related to the degree of injury to the media.