Glucose uptake and homeostasis are regulated mainly by skeletal muscle (SM), white adipose tissue (WAT), pancreas and the liver. Participation of estradiol in this regulation is still under intense investigation. We have demonstrated that, in SM of male mice, expression of the insulin-regulated glucose transporter GLUT4, is reduced by estrogen receptor (ER) agonists. In the present study, to investigate the relative contributions of ER and ER in glucose homeostasis, we examined the effects of tamoxifen (TAM) on GLUT4 expression in SM and WAT in WT and ER-/- mice. ER-/- mice were characterized by fasting hypoglycemia, increased levels of SM GLUT4, pancreatic islet hypertrophy, and a belated rise in plasma insulin in response to a glucose challenge. ER-/- mice, on the contrary, were hyperglycemic, glucose intolerant and expression of SM GLUT4 was markedly lower than in WT mice. TAM had no effect on glucose tolerance or insulin sensitivity in WT mice. In ER-/- mice, TAM increased GLUT4 and improved insulin sensitivity. i.e., it behaved as an ER-antagonist in SM but it had no effect on WAT. In ER-/- mice, TAM did not affect GLUT4 in SM but acted as an ER-antagonist in WAT decreasing GLUT4. Thus in the interplay between ER and ER, ER-mediated repression of GLUT4 predominates in SM, but ER-mediated induction of GLUT4 predominates in WAT. This tissue-specific difference in dominance of one ER over the other is reflected in the ratio of the expression of the two receptors. ER predominates in WAT and ER in SM.