Progesterone is synthesized in the peripheral nervous system in glial cells. The functions of progesterone are indicated by the findings that it stimulates neurite outgrowth from dorsal root ganglia sensory neurones in explant cultures, accelerates the maturation of the regenerating axons in cryolesioned sciatic nerve, and enhances the remyelination of regenerated nerve fibres. The formation of myelin sheaths around axons is a sexually dimorphic process, as the sheaths are thicker in female than in male regenerating nerves. The progesterone-induced myelination is probably mediated by progesterone receptors, as it is impaired by mifepristone (RU486), a progesterone antagonist. The stimulation of neurite growth in the peripheral nervous system may be mediated by a progesterone metabolite, 5alpha-tetrahydroprogesterone, through GABA(A) receptors.