In quail, testosterone (T) activates male copulation and affects the volume and cytoarchitectonic organization of the medial preoptic nucleus (POM). T metabolism (especially its aromatization) is critical for the production of these behavioral effects. We wondered whether T metabolism is also playing a role in the induction of the morphological changes in POM. We compared the effects of T and of its metabolites in this nucleus. To obtain an independent evaluation of the role played by aromatase, morphological effects of T associated or not with the aromatase inhibitor R76713 were also assessed. As previously observed, T increased the POM volume and the cross-sectional area of the neurons in the lateral part of the nucleus. The effects of T on the neurons in the lateral POM were mimicked in part by the combined treatment with estradiol and 5 alpha-dihydrotestosterone. They were also blocked by the aromatase inhibitor. This suggests that T aromatization plays a critical role in the mediation of the cytoarchitectonic effects of T. A specific role for androgens alone remains to be established.