The presence of vertebrate-like steroids, steroidogenic enzymes and steroid receptors has been reported exclusively in cephalopods gonads. The role played by these steroids has been also recently characterized. We here provide the first evidence of steroidogenic activity in the brain of cephalopods and the localization of 3ß-hydroxysteroid dehydrogenase (HSD) activity in the lobes of nervous system of both Sepia and Octopus. Two key steroidogenic enzymatic activity, 3ß-HSD and 17ß-HSD, are present in the nervous system. These activities convert pregnenolone to progesterone and androstenedione to testosterone respectively. Binding experiments seem to assign a functional role to the androgens in the brain of cephalopods. According to the present results, the absence of any progesterone binding moiety supports the hypothesis that progesterone is just a metabolite product along the steroidogenic chain leading to androgens. The presence of these molecules in specific lobes of central nervous system is discussed in terms of the possible role steroids can play in the sexual differentiation of the brain and in the influence of coded behaviours of cephalopods, such as learning processes.