We have previously demonstrated the existence of nuclear estrogen receptors in isolated adipocytes (Pedersen et al. (1991) Biochim. Biophys. Acta 1093, 80-86). In the present study we have investigated the regulatory properties of these nuclear estrogen receptors, in addition to the metabolic effects of estrogen on adipose tissue metabolism. Estrogen treatment (20 μg 17β-estradiol in NaCl for 7 days) decreased lipoprotein lipase activity (LPL) in the adipose tissue by 62% (p < 0.05), decreased adipocyte size by 27% (p < 0.01) and diminished the normal postovariectomy weight gain. Furthermore, estrogen treatment increased the nuclear estrogen receptor binding in adipocytes; in addition, there was a tendency for increased cytosolic estrogen receptor content as well. Time course studies revealed that already 6 h after a single estrogen injection the Bmax increased from 3.82 ± 0.3 fmol/106 cells to 9.8 ± 3.6 fmol/106 cells (p < 0.1) and 24 h after a single injection the Bmax was maximally increased to 12.7 ± 5.5 fmol/106 cells (p < 0.05). The Kd was similar at all time points (about 3-5 nM). Furthermore, the specific insulin receptor binding was increased in adipocytes from estrogen treated rats. The specific insulin binding was maximally increased by 149 ± 6% (p < 0.001) after 4 days of daily estrogen injections. The increased binding seemed to be due to an increased number of insulin receptors on adipocytes from estrogen treated rats with no alteration of the ED50 value. In conclusion it was found that estrogen treatment has a positive feedback effect on its own nuclear receptor. In addition, estrogen treatment was found to have several metabolic effects in the adipose tissue. Estrogen treatment increased the insulin receptor number, decreased LPL activity, increased the lipolytic response in adipocytes and finally decreased body weight and adipocyte cell size.