Steroid hormones are known to act in the central nervous system (CNS), affecting brain development and behavior. They have profound influences on the growth, maturation, differentiation and functioning of brain cells. The biological effects of these steroids are mediated by specific high-affinity intracellular receptors, and their presence in the brain has been described by several groups (for review see McEwen et al., 1982). In earlier studies, we have shown that newborn rat glial cells in primary culture can synthesize pregnenolone and progesterone (P) and that these brain cells contain 4 classes of steroid hormone receptors, progesterone, glucocorticoid, estrogen and androgen receptors (PR, GR, ER and AR). After treating glial cells with estrogen, only the PR was induced, and this induction was most significant in cultures established from female rat pups (Jung-Testas et al., 1991). In agreement with the presence of intracellular steroid hormone receptors, we have shown direct effects of steroids on cell multiplication, morphology and differentiation. For instance, after treatment of cells with P, an increased synthesis of myelin proteins was observed in oligodendrocytes (Jung-Testas et al., 1992). In the CNS, oligodendrocytes are responsible for myelin formation and maintenance. In rodents, central myelin is composed of about 70% lipids, among which galactocerebroside (Gal C) is a specific marker for oligodendrocytes (Raft et al., 1987), and of about 30% proteins, mainly proteolipid protein, myelin