S. mediterranea is a planarian with extensive regenerative ability which requires substantial cell division. However, DNA replication stress occurs every cell division and must be resolved. From yeast to man, an evolutionary conserved DNA replication stress response (RSR) is essential for life. The RSR is a signal transduction pathway regulated by Ataxia telangiectasia and Rad3‐related kinase (ATR) and Checkpoint 1 kinase (CHK1) to respond to replication stress. However, the RSR has not been established in planarian nor do we know if it functions in regeneration. We hypothesize that ATR may be essential for regeneration. Preliminary results show reduced regenerative ability in the presence of hydroxyurea (HU), a replication stress inducing agent. However, low concentrations (2 mM) did not attenuate regeneration, indicating that ATR and the RSR may be present and able to overcome replication stress. To test this, we treated amputated tails with HU, ATR inhibitor (ATRi), and a combination of both. In 2mM HU, tails regenerated similarly to control. We observed only a one day delay in regeneration with ATRi alone. However, ATRi and 2mM HU in combination prevented planarian regeneration completely. This synergistic effect suggests that ATR is significant in the response to replication stress during regeneration. To obtain genetic evidence, we are in the process of knocking down ATR as well as other RSR genes like CHK1, WRN, etc. Further determining the importance of RSR genes in planarian will elucidate the conserved components and allow us to understand how ATR and the RSR contributes to the incredible regenerative abilities in planarian. Support or Funding Information U.S. Department of Education HSI STEM Articulation Grant P031C160118 University of St. Thomas Committee of Student Research This is from the Experimental Biology 2018 Meeting. There is no full text article associated with this published in The FASEB Journal.