Previous experimental evidence suggested that the avian segmental pattern is already specified in the apparently unsegmented paraxial (segmental plate) mesoderm, but is susceptible to modification and reconstitution. We explored capacities of embryos to alter the specified pattern and restore it after disruption. In control experiments, right segmental plates of chicken or Japanese quail embryos were removed after about 48 hours of incubation and immediately replaced. Hensen's node and the primitive streak were removed to halt further segmental plate formation and the embryos were cultured for about 18 hours more. Somite numbers on the operated and unoperated sides were nearly identical (r = 0.904, n = 31, P < 0.001); no species differences were noted. Right segmental plates of chicken hosts were then replaced with right segmental plates from quail donors. The numbers of somites formed by donors and grafts were not significantly correlated (r = 0.305, n = 30, P < 0.1), but the correlation between the graft and the host's unoperated side was significant (r = 0.666, n = 30, P < 0.001). The host is therefore able to alter the number of somites formed by the graft to one more compatible with the host's pattern. From orthostereoscopic reconstructions, it appeared that the location and size of somites could also be adjusted by the host. Similar results were obtained for tandem grafts of anterior halves of segmental plates and for grafts of minced segmental plates, though in the latter case contact with tissues near the midline was necessary for somite formation.