Introduction The observation was made that dehydroepiandrosterone (DHEA) as the unconjugated steroid, and its sulfate ester (DHEAS) are present in the brain of adult male rats (Baulieu 1981, Corpéchot et al. 1981a). This finding was unforseen since rodent steroidogenic glands, including the adrenals, do not secrete significant amounts of DHEA (Vinson et al. 1978). It led to the discovery of a steroid biosynthetic machinery in the nervous system, in charge of producing 'neurosteroids'. We review first the data concerning DHEA characterization, metabolism and levels in the rat brain, and mention observations that are available in other mammalian species. Secondly, we provide an overview of the data concerning the neurobiological effects of DHEA(S); DHEA(S) is used to designate the sum of DHEA and DHEAS. The rat brain DHEA and DHEAS were chromatographed on a Sephadex LH-20 column to completely separate unconjugated steroids from those conjugated to fatty