Many lines of evidence support an involvement of urokinase plasminogen activator (uPA) and its type 1 inhibitor (PAI-1) in the migration of a variety of cells, including normal keratinocytes and carcinoma lines. In the present study, uPA expression was found to be a characteristic not just of migratory but also of proliferative keratinocytes. A variety of naturally occurring and experimentally induced epidermal hyperproliferative conditions were examined in mice, including fetal and neonatal epidermis, tape-stripped epidermis, and epidermis from which the hairs had been gently plucked. In all cases, epidermal hyperproliferation was accompanied by elevated levels of uPA mRNA (as measured by in situ hybridization) and activity (as measured by zymography). uPA mRNA was predominantly localized in the basal and immediately suprabasal cells, which constitute the proliferative population. To determine whether a PAI was concomitantly elevated, in situ hybridization for PAI-1 and PAI-2 was performed. PAI-2 but not PAI-1 mRNA was detected in fetal and neonatal epidermis, localized in the spinous layers. Although mRNAs for both inhibitors were induced by tape-stripping or hair-plucking, their distribution was more focal and more transient than that of uPA mRNA. These findings show that uPA, but not its usual inhibitors, is consistently elevated in the proliferative population of keratinocytes in a diverse range of hyperproliferative states. Two hypotheses are suggested by these data: (a) uPA may play a regulatory role in the activation of epidermal proliferation; or (b) uPA may be involved in the vertical migration of keratinocytes that must accompany increased cell proliferation.