** Shares of advertising company Interpublic Group IPG.N rise 14% to $33.4, while shares of Omnicom Group OMC.N down 4.3% at $99 in premarket trading

** OMC to acquire IPG in a $13.25 billion all-stock deal

** Interpublic shareholders will receive 0.344 Omnicom shares for each share held, or $35.58 based on Omnicom's last close. This represents a premium of 21.6% to Interpublic's close on Friday

** The deal is expected to close in the second half of 2025 and generate annual cost savings of $750 million

** If the merger is done, it may attract regulatory scrutiny as it seeks to combine world's third-largest (based on revenue) ad buyer OMC with the fourth-largest IPG

** Shares of ad firms UK's WPP WPP.L rose 3.2% and France's Publicis Groupe SA PUBP.PA up 1.3%

** Up to last close, IPG stock down ~11%, and OMC stock up ~20% YTD

(Reporting by Priyanka.G in Bengaluru)

((priyanka.g@thomsonreuters.com;))