Internet search leader Google, Inc. (NASDAQ: GOOG) announced late Friday that it agreed to acquire online advertising service DoubleClick Inc. for $3.1 billion in cash.

The deal will combine Google’s powerful text ad network with DoubleClick’s expertise in managing display ads. The deal is expected to close at the end of the year.

It has been our vision to make Internet advertising better - less intrusive, more effective, and more useful. Together with DoubleClick, Google will make the Internet more efficient for end users, advertisers, and publishers, said Sergey Brin, Co-Founder & President, Google Technology.

The purchase represents the largest acquisition in Google's history and comes just six months after Google paid $1.65 billion to purchase video-sharing site, YouTube.

Current owner, San Francisco-based private equity firm Hellman & Friedman and JMI Equity and management, and executives at Google have approved the deal, however it is subject to customary closing conditions.

Google shares were down $1.10. or 0.24 percent, to reach $466.29 at the close of normal trading hours on the Nasdaq Stock Market.