This work summarises data dealing with the biosynthesis of neuroactive steroids (NAS) in the gonads, adrenal cortex and the central nervous system as well as the mechanism of their effects. As far as NAS have been found in different nervous system disorders, epilepsy, alcoholism in women and in reproduction, we speculate that the events are modulated by NAS, having mostly enhancing effects. We investigated and evaluated the correlations of the steroids between the periphery and the cerebrospinal fluid (CSF), considering also the effect of the steroid conjugation in the periphery. It is possible that for the diagnostics of CNS diseases, which in their turn probably are connected with an imbalance in peripheral steroidogenesis (like postpartum depressions, catamenial epilepsy, premenstrual syndrome, affective disorders) it would be much easier to collect peripheral blood, than CSF. This book will be valuable for thousands of those who are interested in neuroendocrinology.