Motor hyperplasia following the enlargement of the periphery by implantation of a supernumerary leg is not due to "remote control" of proliferation, as shown by motor neuron counts in 6-day chick embryos. We have tested the alternative hypothesis that we are dealing with reduction of the naturally occurring cell death. In normal development, the lumbar lateral motor column (l.m.c.) undergoes motor neuron degeneration resulting in a cell loss of at least 40%, which occurs between six and one-half and nine and one-half days. Following transplantation of supernumerary legs, cases selected for vigorous motility showed a numerical difference between experimental and contralateral (control) sides amounting to +11.0% to +27.5%. The transplants were innervated by varying combinations of thoracic and rostral lumbar nerves. We interpret our data in terms of survival of motor neurons which normally would have failed in a competition at the periphery but which were sustained by the enlarged peripheral fields. Our data do not permit a decision between the two alternatives: competition for synaptic sites or for a trophic agent. The surviving motor neurons are not limited to the rostral segments of the motor column but in most instances distributed along its entire rostro-caudal extent, implying a redistribution of all l.m.c. axons. The term "hyperplasia" is no longer appropriate for the phenomenon under consideration and should be replaced by the term "hypothanasia.""