This study was carried out to investigate the nutritional energetics of ovariectomized rats with or without ovarian hormone replacement. Rats were divided into five groups: 1) sham operated, 2) ovariectomized, 3) ovariectomized and treated with progesterone, 4) ovariectomized and treated with estradiol, or 5) ovariectomized and treated with estradiol and progesterone. After 36 days of treatment, energy contents of all five groups were determined together with energy content of food and feces. Brown adipose tissue thermogenesis was assessed through mitochondrial GDP binding assay. Results show that ovariectomy leads to a 16% increase in metabolizable energy intake. This increase was accompanied by a twofold increase in body energy gain. Progesterone did not further affect energy intake and gain in ovariectomized rats. However, increases in both food intake and energy gain were prevented by the estradiol replacement therapy. There was no difference in energy expenditure between sham-operated and ovariectomized rats in the absence of estradiol. In estradiol-treated animals, energy expenditure (kJ.kg body wt-0.75 . day-1) showed a slight increase. There was no difference in protein content of interscapular brown adipose tissue between all five groups. GDP binding was slightly reduced in ovariectomized estradiol-treated rats. It is concluded from this study that ovarian hormones produce their effects on energy balance mainly by altering food intake.