It is now well established that some serine proteases, such as plasminogen activators and thrombin, as well as their inhibitors, have roles in the development of both central and peripheral nervous systems. We have shown that muscle plasminogen activators, activated after denervation, were able to digest some components of the muscle basement membrane. We have also shown that several inhibitors of serine proteases were concentrated at the neuromuscular junction. These are protease nexin, I, also called glia-derived nexin, protease nexin II, a secreted form of the beta-amyloid precursor protein (APP), and alpha 1-antichymotrypsine (ACT). These results leads us to propose a model in which serine proteases would favor plasticity of motor nerve endings during neuromuscular development. On the contrary, the inhibitors of serine proteases would act to provide and secure maintenance of the synaptic contact. A dysequilibrium between serine proteases and their inhibitors might underlie one or more motor neuron diseases.