The regeneration of structures in adult animals depends on a mechanism for coupling the acute response to tissue injury or removal with the local activation of plasticity in residual differentiated cells or stem cells. Many potentially relevant signals are generated after injury, and the nature of this mechanism has not been elucidated for any instance of regeneration. Lens regeneration in adult vertebrates always occurs at the pupillary margin of the dorsal iris, where pigmented epithelial cells (PEC) reenter the cell cycle and transdifferentiate into the lens, but the basis of this striking preference for the dorsal margin over the ventral is unknown. In this study, we report that a critical early event after lentectomy in the newt is the transient and selective activation of thrombin at the dorsal margin. The thrombin activity was blocked with two different irreversible inhibitors and was shown to be strictly required for cell cycle reentry at this location. The axolotl, a related urodele species, can regenerate its limb, but not its lens, and thrombin is activated in the former context, but not the latter. Our results indicate that selective activation of thrombin is the pivotal signal linking tissue injury to the initiation of vertebrate regeneration.