Although estrogen is thought to protect the skeleton by inhibiting bone resorption, we have also found that in the rat, estrogen stimulates cancellous bone formation. However, the extent to which the various skeletal actions of estrogen are mediated by classical estrogen receptors remains unclear. Although estrogen receptor antagonists such as tamoxifen have been used to study this question, interpretation of the results is complicated by the fact that this agent also acts as a partial estrogen agonist. However, the recent development of estrogen antagonists devoid of agonist activity provides an opportunity to explore this question further. We, therefore, investigated the effect of administration of the pure estrogen antagonist ICI 182,780 on the skeleton of adult female rats. We found that ICI 182,780 reduced bone volume at the proximal tibial metaphysis by approximately 30%, associated with an increase in osteoclast surface. We then investigated the effect of ICI 182,780 on the anabolic action of estrogen. We used ovariectomized rats treated with 3-amino-1-hydroxypropylidene-1-bisphosphonate to inhibit bone resorption, thereby preventing any increase in bone formation as a result of the stimulation of bone resorption due to estrogen deficiency. 17 beta-Estradiol (1 micrograms/kg) stimulated cancellous bone formation in such animals by approximately 8-fold; this increase was abolished when ICI 182,780 was also given. In contrast, ICI 182,780 affected neither longitudinal nor periosteal tibial growth in either intact animals or ovariectomized rats given estradiol or vehicle. We conclude that ICI 182,780 reduces cancellous bone volume in the rat by antagonizing estrogen's actions on bone formation and resorption, suggesting that these processes are both mediated by classical estrogen receptors.