The stock hit its previous high of $299.59 on June 7.
Google shares have more than trebled in value since the company's initial public offering in mid-August. Analysts and traders gave a variety of potential explanations for the uptick.
"There was a pretty large (consensus) among professional traders that the stock would have trouble breaching $295 to $300 level. But today's action proved them wrong," said Tim Biggam, chief options strategist at Man Securities, a Chicago-based options brokerage firm.
"I think we are seeing some short covering in both the share prices and short call positions on Google. People who had thought Google may go up but not pass $300 a share are now scrambling to buy back their short call positions," Biggam added.
Paul Foster, strategist at financial-information Web site Theflyonthewall.com, offered another view, saying the shares may have been lifted by end-of-quarter buying known as "window dressing"--when money managers buy shares to boost their portfolio.
Benjamin Schachter, Internet analyst at UBS Securities said he thinks the shares, which debuted at $85 each, have not yet topped out.
"We have a $350 target. I think this thing still has room to run. When you a have a company that blows out numbers each quarter the expectation it will continue to do well is not particularly unrealistic," he said.
Google in April posted a first-quarter profit that was, blowing by Wall Street targets.
Other high-flying Internet names, including eBay have split their stocks after big price gains, but a similar move by Google does not appear to be imminent.
"They have given zero indication. They seem to be taking a page from Warren Buffett and not doing that," Schachter said, referring to shares of Buffett's Berkshire Hathaway, which closed at $83,500 on the New York Stock Exchange on Monday.
Eric Schmidt, Google's Chief Executive, said in mid-May that the company had no plans to split its stock, which at the time was trading at around $230 per share.
Google--which recently ousted eBay as the Internet's most valuable name with a market valuation of more than $87 billion--now trades at a multiple of 48 times analysts' 2005 projected earnings per share.
eBay's comparable multiple is 46, Yahoo's is 62 and Amazon's is 56.