Changes in bone mass and cancellous bone remodeling parameters were determined during lactation in adult beagles. Bone mineral content was determined by photon absorptiometry, fraction of mineralized cancellous bone area and perimeter to area ratios by image analysis of microradiographs, and bone remodeling parameters by fluorochrome-based histomorphometry. Bone mineral content of the intact proximal humerus and fraction of mineralized tissue areas of lumbar vertebral cancellous bone were decreased during lactation when compared with controls. There were significant increases during lactation in osteoid volumes and osteoid surface, although osteoid seam thickness was not changed. There were significant increases in the fraction of fluorochrome-labeled cancellous bone surfaces in the lactating dogs when compared with controls. The mineralizing surface, bone formation rate and activation frequency were significantly increased in the lactating dogs when compared with controls. There were also significant increases in resorption parameters during lactation. These data indicate that during lactation, bone remodeling and bone turnover is increased. Increased bone remodeling during lactation in a longer-lived mammal may represent an example of a "reversible mineral deficit" which may be an important physiological mechanism in nursing mothers to ensure adequate calcium for milk production. Increased bone remodeling may also provide a physiological mechanism to enhance the capacity of the skeleton to better accommodate the greatly increased and episodic changes in mineral homeostasis during lactation.