NEW YORK (CNNMoney.com) - Google stock posted its largest single-day loss ever Friday as investors showed concern about prospects for the Internet advertising market and the company's role in a Justice Department lawsuit about Internet searches.

Shares in Google (Research) sank $36.98 to $399.46 -- about 8.5 percent -- in heavy trading on Nasdaq. Volume of 31 million shares was nearly triple the normal daily average.

The loss was the largest in both dollar and percentage terms. The shares previously lost 8.31 percent on Nov. 5, 2004. The previous record for dollar decline was $22.20 on Jan. 18, 2006.

Investors sent Yahoo tumbling Tuesday after the largest Internet media company surprised investors by posting earnings below expectations. (Full story)

Google said Thursday it was resisting a Justice Department subpoena that the company turn over search records for a case regarding an anti-pornography law. (Full story)

Google's stock has had a meteoric rise in its short time as a public company. The company initially offered its shares to the public at $85 on Aug. 19, 2004.

The stock reached an all-time closing high of $471.63 on Jan. 11, 2006.

Brokerage Sifel, Nicolaus & Company downgraded the stock to 'Sell' from 'Hold' this week, believing it is overvalued.

The analysts noted that Google's upward momentum and positive sentiment may be evaporating and that Yahoo's disappointing results will force investors to make a more realistic appraisal of the Internet search and advertising sector's prospects.

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Major stock indexes tumbled Friday -- full story here.

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