Conflicting reports of distal phalangeal regrowth prompted a reexamination of bone growth following phalangeal amputation in mammals. Digits of neonatal and adult mice and rats were amputated at various levels. The short-term response was examined on histological sections, and long-term growth was documented by alizarin red-staining of KOH-digested digits. Three patterns of response were seen to correspond to three general levels of amputation. Complete bone regeneration occurred frequently by five weeks following amputation through the distal one-quarter of the distal phalanx. Amputation through the central region of the distal phalanx yielded substantial bone growth, but the form of the regrowth was imperfect even three months after amputation. Amputation through more proximal levels of the digit yielded no significant elongation. To investigate why the response varies in relation to the level of amputation, we are conducting both in vivo and in vitro experiments. We have learned that simple avulsion of the nail plate provokes substantial remodeling of the distal phalanx. We are further exploring the trophic influence of nail organ on bone structure and growth in vivo. We have also recently determined that entire digits may be kept alive in vitro when cultured in DMEM:F-12:BGJb medium supplemented with insulin, EGF and FGF. This system sufficiently replicates in vivo conditions such that osteogenesis occurs both endosteally and distal to the amputation plane in vitro. The effects of growth factors, retinoic acid, and the presence or absence of nail organ components on amputational bone growth at all three levels are currently being studied in vitro. The goal of these studies is to determine why bone fails to grow, undergoes hyperplasia, or regenerates following amputation at different levels in mammals.