Regeneration processes in many tissues are modulated by various factors, which are involved in their organogenesis. Activin A, a member of the TGF-beta superfamily, inhibits branching tubulogenesis of the kidney in organ culture system as well as in in vitro tubulogenesis model. On the other hand, follistatin, an antagonist activin A, reverses the effect of activin A on kidney development, induces branching tubulogenesis, and also promotes tubular regeneration after ischemia/reperfusion injury by blocking the action of endogenous activin A. The activin-follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidneys.