The transformation of testosterone into estradiol in the brain plays a key role in several behavioral and physiological processes, but it has been so far impossible to localize precisely the cells of the mammalian brain containing the relevant enzyme, viz., aromatase. We have recently established an immunohistochemical technique that allows the visualization of aromatase-immunoreactive cells in the quail brain. In this species, a marked increase in the optical density of aromatase-immunoreactive cells is observed in subjects that have been treated with the aromatase inhibitor, R76713 or racemic Vorozole. This increased immunoreactivity, associated with a total blockade of aromatase activity, has been used as a tool in the present study in which the distribution of aromatase-immunoreactive material has been reassessed in the brain of mice pretreated with R76713. As expected, the aromatase inhibitor increases the density of the immunoreactive signal in mice. Strongly immunoreactive cells are found in the lateral septal region, the bed nucleus of the stria terminalis, the central amygdala, and the dorso-lateral hypothalamus. A less dense signal is also present in the medial preoptic area, the nucleus accumbens, several hypothalamic nuclei (e.g., paraventricular and ventromedial nuclei), all divisions of the amygdala, and several regions of the cortex, especially the cortex piriformis. These data demonstrate that, contrary to previous claims, aromatase-immunoreactive cells are present in all brain regions that have been shown previously to contain high aromatase activity.