Tartrate-resistant acid phosphatase (TRACP) and carbonic anhydrase II (CA II) are key enzymes responsible for osteoclastic bone resorption. In this study, we proposed that estrogen loss in postmenopausal osteoporosis may enhance gene expression of TRACP and CA II, and subsequently increase osteoclastic bone resorption. We have, therefore, used the ovariectomized rat model of postmenopausal bone loss to investigate changes at the gene transcriptional level in osteoclastic bone-resorbing enzymes in ovariectomized (OVX) rats, sham ovariectomized (S-OVX) rats, and estrogen-treated ovariectomized (E-OVX) rats. We have demonstrated for the first time that ovariectomy in rats enhances gene expression of TRACP, and CA II. The mRNA levels in OVX were approximately three- and four-fold higher, respectively, than those in S-OVX. Enhancement was observed 1 week after ovariectomy and transcripts remain high during the experimental period of 8 weeks. Administration of 17 beta-estradiol to OVX (E-OVX) reduced gene expression of these osteoclastic bone-resorbing enzymes 18 hours after injection. It appeared that the suppression of the osteoclastic bone-resorbing enzymes by 17 beta-estradiol was most effective during the first 1-2 weeks but the degree of suppression was reduced at 8 weeks after ovariectomy. In conclusion, our results suggest that estrogen prevents bone loss by reducing the mRNA levels of osteoclastic bone-resorbing enzymes in bone tissue.