We have previously described a positive correlation between poly(ADP-ribosyl)ation capacity of mononuclear blood cells with longevity of mammalian species. Our comparison of purified recombinant human and rat PARP-1 revealed that this correlation might be explained in part by evolutionary sequence divergence. We have also developed molecular genetic approaches to modulate the poly(ADP-ribosyl)ation status in living cells. Our results revealed that PARP-1 acts as a negative regulator of DNA damage-induced genomic instability, as assessed by sister-chromatid exchange and micronucleus formation. Genomic instability is considered an important driving force for carcinogenesis as well as for the ageing process. We recently addressed the role of poly(ADP-ribosyl)ation in the maintenance of telomeres in two telomerase-positive cell systems. Telomeres are repetitive sequences protecting the ends of chromosomes against degradative and inappropriate repair processes. We monitored telomere length by quantitative fluorescence in-situ hybridization (Q-FISH) in SV40-transformed embryonic hamster cells and in HeLa cells as a function of time after implementation of PARP inhibition by 3-aminobenzamide exposure or overexpression of the PARP-1 DNA-binding domain as well as after release from PARP inhibition. We observed that PARP inhibition in SV40-transformed embryonic hamster cells or in HeLa cells by 3-aminobenzamide treatment leads to a rapid but reversible decline in telomere length in a dose-dependent fashion. Telomere length stabilised at about 70% of the original length. Rapid and reversible telomere shorting was also observed in hamster cell clones, in which poly(ADP-ribose) formation was suppressed by overexpression of the PARP-1 DNA-binding domain, thus causing trans-dominant PARP inhibition. Telomere loss was faster under 3-aminobenzamide, amounting to about 300 bp per cell division, which exceeds the estimates for DNA replication-induced telomere loss in telomerase-negative cells. We conclude that poly(ADP-ribose) formation contributes to effective telomere maintenance in telomerase-expressing cells, thus perhaps delaying cellular senescence or apoptosis induced by critically short telomeres.