Maternal efficiency during reproduction is essential for species propagation. The mammalian skeleton provides mineral for the fetal skeleton during pregnancy and milk production during lactation. Dramatic changes occur in the maternal skeleton during the reproductive cycle and some may serve to protect skeletal integrity. The post-lactation period is particularly 'anabolic', but mechanistically not understood. To further define the maternal skeletal adaptations and mechanisms that occur during and after the reproductive cycle, the following aims are proposed. 1). To determine the cellular and tissue events in the skeleton during the transition from late pregnancy to lactation. We hypothesize that the increases in resorption in late pregnancy and sustained during lactation are the result of an increased rate of osteoclast formation, prolongation of lifespan and a decrease in apoptosis. 2) To determine the cellular and tissue events in the skeleton that occur during the transition from lactation to post-weaning. The hypothesis is that increases in bone formation are associated with an increased rate of osteoblast formation but decreased resorption is the result of decreased osteoclast activity, decreased formation but increased apoptosis. 3) To determine if the post-weaning increase in calcitonin (CT) levels is correlated with decreased osteoclastic functions, and establishes the "milieu" for the post-lactational recovery phase. We suspect correlation with decreases in prolactin, but increased estrogen and initiation of estrus. 4) Is there a change in the remodeling period during the reproductive cycle in mammals? This will be determined from archived materials from a species that has intracortical bone remodeling. 5) Does moderate skeletal loading improve bone mass, structure, strength and dynamics during the post-lactation 'recovery' phase? Our hypothesis is that skeletal loading will have synergistic effects on skeletal mass and structure during this period. These studies will provide new knowledge on perhaps the most dynamic and "anabolic" period in the life of the adult female skeleton and may provide new insights into treatments of diseases after reproductive capacity has ended.