Nitric oxide (NO) has been reported to inhibit osteoclastic bone resorption. We examined the bone-sparing effect of NO after 6 weeks of administration into estrogen-deficient rats. 30 female Sprague-Dawley rats, 12 weeks ofage, underwent ovariectomy (OVX), and 5 rats were sham-operated. OVX rats were assigned to six groups (n = 5/group) treated respectively with[ vehicle[ 17-β-estradiol (E₂)[ nitroglycerine (NG, NO donor)[ N^G-nitro-l-arginine methyl ester (L-NAME, NO synthase inhibitor)] combination of E₂ + NG[ and a combination of E₂ + L-NAME. Prior to treatment and at the end of the treatment period, bone mineral density (BMD) of rats was measured by dual-energy X-ray absorptiometry (DXA) scanning. OVX animals had significantly lower BMD and femur weights in comparison to sham operated rats (p < 0.01), and this was completely prevented by the administration of E₂ (p < 0.01). Administration of NG alone prevented OVX-induced bone loss (p < 0.05). The combination of E₂ + NG did not further enhance the bone mass or femur weight, and the OVX-induced bone loss was not further aggravated by L-NAME. However, in the presence of L-NAME, E₂ was totally ineffective in reversing the bone loss, suggesting that the protective effect of estrogens against bone loss may be mediated through NO. In summary, the results suggest that NO counteracts the bone loss associated with OVX.