Human placental antigen X-P2 (hPAX-P2), an antigen complex associated with cytochrome P-450 of aromatase within estrogen synthesizing tissues, has been reported to be present in a distinct group of rat primary olfactory receptors involved in suckling behavior. In this study, most of the mitral and tufted cells in the rat olfactory bulb were found to possess hPAX-P2 immunoreactivity. This suggests that the activity of these cells can be hormonally modulated and that hPAX-P2 is involved in rat olfaction not only at the receptor level but also at integrative brain levels via the secondary projecting neurons of the olfactory pathway.