The role of estrogen in bone resorption has been specifically related to the effect of estrogen on the signalling pathway that inhibits the formation of osteoclasts. However, osteoclast apoptosis and a significant reduction in the number of these cells have been observed in the alveolar bone of female rats treated with estradiol. In the present study, the expression of estrogen receptor b (ERb) in the cells of alveolar bone was evaluated in estradiol-treated and -untreated female rats. In order to test the possible direct action of estrogen on osteoclasts, the relationship between apoptosis and ERb expression in these cells was also analysed. The animals received estradiol for 14 days and the alveolar bone fragments were embedded in paraffin for the quantification of tartrate-resistant acid phosphatase-positive osteoclasts. The expression of ERb and apoptosis in the osteoclasts were evaluated by ERb immunohistochemistry and Terminal deoxynucleotidyl transferase-mediated dUTP NickEnd Labelling (TUNEL) methods, respectively. To confirm osteoclast death by apoptosis, these cells were analysed under transmission electron microscopy. Some osteoclasts from estradiol-treated animals were found to be undergoing apoptosis and the number of tartrate-resistant acid phosphatase-positive osteoclasts was significantly reduced. ERb immunolabelling was observed in the cytoplasm and nuclei of active osteoblasts, osteocytes and osteoclasts in both groups, suggesting a direct participation of estrogen on alveolar bone cells. However, following estradiol treatment, a strong ERb immunolabelling was often observed in the TUNEL-positive osteoclasts. Therefore, these results indicate that, in addition to the other signalling pathway, the reduction of alveolar bone resorption is also related to a direct action of estrogen on osteoclasts, promoting apoptosis in these cells, via ERb.