estrogen binding sites in hamster white adipose tissue sex and siterelated variations modulation by testosterone estrogen binding sites er were studied using 17 beta3hestradiol as the ligand in epididymal or parametrial and subcutaneous adipose tissues of male and female hamsters compared with other mammalian fat deposits intact male and female hamsters possess abundant estrogen binding sites with moderate affinity for estradiol and which occur as a single class of receptor in males but as two populations in females the levels of estrogen receptors depend on both sex and tissue localization in males receptor densities are higher in both localizations when compared to those of females and er are more abundant in superficial adipose deposits than in the deep fat tissue in females there are two estrogen binding populations the one with the highest affinity is similar to the classical estrogen receptor and both populations are more abundant in deep fat than in subcutaneous deposits in contrast to male hamsters these characteristics depend on androgen status in male adipose tissues testosterone tp upregulates the er levels conversely in female fat deposits tp downregulates the highest affinity estrogen receptors and the lowest affinity population disappears binding affinities are never affected by testosterone these results suggest that in hamster adipose tissue estrogen receptors exhibit site and sexrelated differences as previously described for androgen receptors furthermore estrogen receptor expression is modulated by the androgen status depending on gender which could be related to some physiological situations observed in the hamster