Regenerative medicine aims to replace lost or damaged tissues and organs with functional parts of the correct size and shape. To achieve this goal, we need to understand what determines the scale and form of regenerating tissues. Michael Levin and colleagues have been tackling this issue by investigating the regulation of organ size during planarian regeneration (see p. 313). During this process, existing tissues are remodelled concurrently with new tissue growth to maintain the correct relative tissue proportions. The researchers show that, in Schmidtea mediterranea, membrane voltage-dependent bioelectric signalling determines head size and organ scaling during regeneration. Specifically, RNA interference of the H+,K+-ATPase ion pump causes membrane hyperpolarisation and produces regenerated animals with shrunken heads and oversized pharynges, but does not inhibit the production of new tissue (blastema). Other experiments indicate that the H+,K+-ATPase ion pump maintains proportionality during regeneration by mediating apoptotic sculpting of the original tissues. Thus, bioelectric signalling regulates the cellular mechanisms that control organ size and shape during regeneration.