The regenerating amphibian jaw represents an important model for studying pattern formation and the mechanisms underlying regeneration of facial structures. We have studied regeneration of upper and lower jaws in the urodele amphibian, Notophthalmus viridescens, using whole mount preparations stained for bone and cartilage, scanning electron microscopy and immunocytochemistry to further characterize these regenerating systems. In addition, we have investigated whether lower jaws of adults and larvae display similar regenerative ability. Although in adult animals the original shape of both the lower and upper jaws is rather faithfully reproduced following amputation, and the teeth and oral mucosa with its specialized sensory organs fully regenerate, significant differences in the regenerative ability of the various skeletal elements are observed. In fact, only tooth-bearing skeletal elements ossify, while the other elements of the regenerated skeleton remain cartilaginous for as long as 5 months after amputation. In contrast, a regenerated lower jaw in the larva is indistinguishable from an unamputated one at the same stage of development. Interestingly, regenerating adult jaws form directly bicuspid teeth, which are the type of teeth normally found in the adult, rather than the monocuspid teeth characteristic of larval jaws, indicating that jaw regeneration is not a recapitulation of development, in that an adult jaw blastema directly regenerates an adult jaw. Finally, we have studied the expression of tissue specific markers in normal and regenerating upper and lower jaws to establish whether the blastemal cells, which will form the missing part of the jaw, express any of these markers of the differentiated state, or are undifferentiated as suggested by their morphological appearance. Under our experimental conditions, no expression of markers of the differentiated state, such as those for muscle, cartilage and glands is detectable in early regenerates. On the contrary, the mesenchymal marker 22/31, whose expression in normal jaws is restricted to dermal fibroblasts and the dental pulp, is expressed in at least a half of the blastemal cells. The significance of these observations in relation to the origin of blastemal cells in the jaw will be discussed.