Because planarians can regenerate a complete body from a tissue fragment they present a powerful system in which to study cell, tissue and organ regeneration, and to look for conserved developmental mechanisms. Peter Reddien and colleagues now describe a regulatory programme for the regeneration of the planarian Schmidtea mediterranea excretory system, the protonephridia (see p. 4387). The S. mediterranea protonephridia consists of tubules, which are dispersed throughout the animal's body, two types of tubule-associated cells, and ciliated terminal cells, which drive filtration from the extracellular space into the tubule lumen. The researchers use RNAi screening assays and microarray analyses to show that Six1/2-2, POU2/3, hunchback, Eya, Sal1 and Osr, which encode transcriptional regulators, are involved in protonephridia regeneration. Notably, apart from hunchback, all these genes are also required for vertebrate kidney development. Moreover, the researchers show that planarian and vertebrate excretory cells express several homologous proteins involved in reabsorption and waste modification. Together, these findings suggest that metazoan excretory systems share a common evolutionary origin.