We have analysed the recircularisation of plasmid DNA, cut with two different endonucleases to generate non-homologous DNA ends, in extracts of unfertilised eggs and oocytes of Xenopus. We found that the capacity to join non-homologous DNA ends, generating diagnostic covalently closed monomer circles, appeared during oocyte maturation at the time of germinal vesicle breakdown. This enzyme function was post-translationally activated in oocyte extracts incubated with unfertilised egg extract containing active cdc2kyclin B, or by incubation with purified cdc2/cyclin B. Dephosphorylation of egg proteins by alkaline phosphatase inhibited the ability to join non-homologous DNA endr. We show that most linear non-homologous DNA ends repaired to form closed-circular supercoiled monomers, are joined without loss of nucleotides. Following partial purification, the activity was inhibited by inhibitors of poly(ADP-Rib) polymerase, an enzyme that is inactive in oocytes, but phosphorylated and activated during maturation. Competitive inhibition of poly(ADP-Rib) polymerase by > 50 pM 3-aminobenzamide prevented the joining of both matched and non-homologous DNA ends. We conclude that post-translational phosphorylation provides one route by which end-joining of non-homologous DNA can be regulated.