Providing enough calcium for milk production stresses calcium homeostasis in lactating mammals. A universal response to these demands for calcium appears to be the mobilization of maternal skeletal reserves, and bone loss during lactation has been well documented. However, the regulation of calcium and skeletal metabolism during lactation remains enigmatic. Our study was designed to examine mineral and bone metabolism in lactating mice. We found that mice lose bone rapidly at all sites during lactation. Bone mineral density as determined by dual-energy x-ray absorptiometry was 20 to 30% lower at the spine, femur, and total body in lactating compared with either age-matched virgin or pregnant mice. The decrease in bone mineral density was accompanied by dramatic reductions in bone volume and changes in trabecular architecture. Bone loss was also accompanied by increases in bone turnover as determined by biochemical markers and histomorphometry. PTHrP levels were elevated during lactation and correlated positively with markers of bone resorption and negatively with bone mass at all sites. Estrogen levels were low during lactation and correlated negatively with bone resorption markers. Finally, estrogen and pamidronate treatment lowered rates of bone resorption to baseline virgin levels and mitigated, but did not prevent, bone loss. These data suggest that the combination of estrogen deficiency and elevations in circulating PTHrP during lactation act to stimulate bone resorption and promote bone loss.