Aromatase-containing neurons were immunohistochemically examined in rat brains by using a polyclonal antibody against human placental antigen. The antibody recognizes cytochrome P-450 portion of aromatase, an enzyme converting androgen to estrogen. A large group of strongly immunoreactive cells was identified in the ventral pallidum, which extends caudally from the area surrounding the islands of Calleja. Other strongly or moderately stained cell groups were observed in the cerebral cortex, the amygdaloid area, the nucleus of the diagonal band, and the area anterior to the posterior commissure. Only a few stained cells were present in the medial preoptic region. These findings cast doubt upon the previous assumption, based on biochemical analysis of tissue samples, that the center of the aromatizing system is in the medial preoptic region. They indicate instead that most aromatase-containing neurons of rats lie within the ventral pallidum ventromedially adjacent to the preoptic area.