Protease Nexin-1 (PN-1) also known as Glia-Derived Nexin (GDN) inhibits the activity of several serine proteases including thrombin, tissue (tPA)- and urokinase (uPA)-type plasminogen activators. These and other serine proteases seem to play roles in development and tissue homeostasis. To gain insight into where and when PN-1 might counteract serine protease activities in vivo, we examined its mRNA and protein expression in the mouse embryo, postnatal developing nervous system and adult tissues. These analyses revealed distinct temporal and spatial PN-1 expression patterns in developing cartilage, lung, skin, urogenital tract, and central and peripheral nervous system. In the embryonic spinal cord, PN-1 expression occurs in cells lining the neural canal that are different from the cells previously shown to express tPA. In the developing postnatal brain, PN-1 expression appears transiently in many neuronal cell populations. These findings suggest a role for PN-1 in the maturation of the central nervous system, a phase that is accompanied by the appearance of different forms of PN-1. In adults, few distinct neuronal cell populations like pyramidal cells of the layer V in the neocortex retained detectable levels of PN-1 expression. Also, mRNA and protein levels did not correspond in adult spleen and muscle tissues. The widespread and complex regulation of PN-1 expression during embryonic development and, in particular, in the early postnatal nervous system as well as in adult tissues suggests multiple roles for this serine protease inhibitor in organogenesis and tissue homeostasis.