Estrogen formation in the brain catalysed by the cytochrome P450arom is required for the control of estrogen-dependent neural mechanisms regulating reproductive behaviour. A polyclonal antibody was raised against a 15-amino acid fragment of the chicken ovarian P450arom protein, to localise aromatase-immunoreactive (AR-IR) cells in the adult female chicken brain. Specificity of antibody reaction was established by Western blot and by inhibition of aromatase activity in homogenates of chicken ovarian follicles determined by a radiometric assay. The AR-IR material in the brain was localised in the perikarya and some of their adjacent cytoplasmatic processes. Intense immunoreactivity was observed in the preoptic region as well as in other hypothalamic nuclei. AR-IR cells were also found in extrahypothalamic areas; in particular, in the area entorhinalis and hippocampus. These results confirm histologically that aromatization of testosterone in the adult female chicken brain occurs in preoptic nuclei closely associated with the regulation of reproductive behaviour. The mapping of AR-IR cells in the female chicken brain now allows study of its regulation under different physiological and environmental conditions, and its relation to classic target areas expressing estrogen receptors.