Adipose tissue is responsive to estrogen and expresses both estrogen receptor alpha and beta. To test the hypothesis that the estrogenic soy isoflavone genistein can have effects on adipose tissue, juvenile or adult C57/BL6 mice were ovariectomized and given daily injections of vehicle, 17beta-estradiol (5 microg/kg.d) or genistein (8-200 mg/kg.d) sc for 21-28 d. To test effects of dietary genistein, 25- to 27-d-old mice were fed diets containing 0-1500 parts per million (ppm) genistein for 12 d. Mice were killed and fat pads weighed. Parametrial fat pads were used for morphometric and Northern analysis. Genistein injections decreased adipose weight and adipocyte circumference at higher doses; effects in adult and juvenile mice were similar. Genistein decreased lipoprotein lipase mRNA, which may be a critical aspect of its adipose effects. Juveniles fed 500-1500 ppm dietary genistein had dose-responsive decreases in fat pad weights of 37-57%, compared with controls; 300 ppm genistein did not cause decreases. Genistein doses of 300, 500, 1000, and 1500 ppm produced serum genistein concentrations of 1.02 +/- 0.14 microM, 1.79 +/- 0.32 microM, 2.55 +/- 0.18 microM, and 3.81 +/- 0.39 microM, respectively. These results indicate dietary genistein at 500-1500 ppm produces antilipogenic effects in mice at serum levels that humans are realistically exposed to.