Plasminogen activators and inhibitors may be important early in primate implantation but evidence for this is sparse in non-human primates. We define the expression of urokinase type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), plasminogen activator inhibitor type 1 (PAI-1) and type 2 (PAI-2), the receptor for uPA (uPAR) and fibrin/fibrinogen in monkey implantation sites. In situ hybridization and immuno-histochemical localization of rhesus monkey implantation sites (day 15-16 postovulation) indicate: (1) uPA mRNA is localized to placental trophoblast, epithelial plaque and endometrial stroma. (2) tPA mRNA is mainly expressed in glandular cells of endometrium. (3) PAI-1 expression is linked to a specific population of trophoblasts that confront maternal cells, adding support to our view that it has a regulatory role in trophoblast invasion. (4) Localization of tPA antigen confirms that uterine glands are the major source of tPA and that it is also closely associated with fibrin(ogen) suggesting its possible function during implantation is fibrinolysis. (5) Unlike uPA mRNA, however, the distribution of uPA protein and its cell surface receptor uPAR suggests that it mediates trophoblast invasion and plays a significant role in angiogenesis. (6) PAI-2, the inhibitor associated with pregnancy in humans, was found in unidentified cells located specifically along the maternofetal junction. This localization adjacent to areas of cell death at the maternofetal junction implies that it may have a role as a protective curtain with anti-apoptotic function. In conclusion our results suggest that gene expression of PAs and PAIs in early implantation sites are tissue-specific, location-sensitive and function-related. (C) 2001 Harcourt Publishers Ltd.