It is shown here that amputated and denervated limbs of larval axolotls dedifferentiate and a proportion of the cells released undergo DNA synthesis and mitosis. When the limb is denervated prior to amputation fewer cells go through the cell cycle, implying the existence of a pool of trophic factor in the limb. Recent work has demonstrated that denervated blastemal cells accumulate in the G1 phase of the cycle. These results strongly argue against the theory that the trophic factor controls the G2 phase. Rather, it is proposed that this factor regulated either the total number of cells cycling or the rate at which they cycle by varying the length of the G1 phase.