The central nervous system (CNS) and the peripheral nervous system (PNS) are targets for steroid hormones where they regulate important neuronal functions. Some steroid hormones are synthesized within the nervous system, either de novo from cholesterol, or by the metabolism of precursors originating from the circulation, and they were termed 'neurosteroids'. The sex steroid progesterone can also be considered as a neurosteroid since its synthesis was demonstrated in rat glial cell cultures of the CNS (oligodendrocytes and astrocytes) and of the PNS (Schwann cells). Both types of glial cells express steroid hormone receptors, ER, GR and PR. As in target tissue, e.g. the uterus, PR is estrogen-inducible in brain glial cell cultures. In the PNS, similar PR-induction could not be seen in pure Schwann cells derived from sciatic nerves. However, a significant PR-induction by estradiol was demonstrated in Schwann cells cocultured with dorsal root ganglia (DRG), and we will present evidence that neuronal signal(s) are required for this estrogen-mediated PR-induction. Progesterone has multiple effects on glial cells, it influences growth, differentiation and increases the expression of myelin-specific proteins in oligodendrocytes, and potentiates the formation of new myelin sheaths by Schwann cells in vivo. Progesterone and progesterone analogues also promotes myelination of DRG-Neurites in tissue culture, strongly suggesting a role for this neurosteroid in myelinating processes in the CNS and in the PNS.