The ultrastructure of aromatase-associated “stigmoid (dot-like) structures,” which were detected in a previous study using light-microscopic immunohistochemistry (Shinoda et al.: J. Comp. Neurol. 322: 360–376,'92), were examined in the rat medial preoptic region, bed nucleus of the stria terminalis, medial amygdaloid nucleus, and arcuate nucleus by pre-and post-embedding marking with a polyclonal antibody against human placental antigen XP 2 (hPAX-P 2) for immuno-electron microscopic analysis. The immunoreactive stigmoid structure was identified as a distinct, non-membrane-bounded cytoplasmic inclusion (approximately 1–3 μm in diameter), which has a granulo-fuzzy texture with moderate-to-low electron density in non-immunostained preparations. It consists of at least four distinct granular and three distinct fibrillo-tubular elements forming a granulo-fibrillar conglomerate. This type of inclusions was formally termed the “stigmoid body” under the electron microscope. The stigmoid body is composed of the outer granulo-fibrillar and inner hyaloplasmic compartments. The immunoreactivity for hPAX-P 2 is mainly localized to the former, especially to the low density granulo-fuzzy materials associated with the fibrillo-tubular elements. Identification of the ultrastructure of stigmoid body clarified their prevalence not only in the limbic and hypothalamic regions, but also in sex-steroid-sensitive peripheral tissues (eg, peripheral sensory ganglia, ovary, testis) by consulting earlier electron-microscopic studies.