MOUNTAIN VIEW, Calif. - Google Inc. has started testing a long-awaited radio advertising service that represents the Internet search leader's most elaborate attempt yet at expanding its financial clout beyond the Web.

The test announced Thursday will help sell advertising on more than 700 radio stations in more than 200 US metropolitan markets. Google hopes to eventually sign up more than 5,000 stations, according to documents shown potential advertisers.

For now, at least, Google will lag well behind other radio advertising placement services like Softwave Media Exchange, which says it has enlisted more than 1,500 stations with a combined daily audience of more than 9 million listeners.

Thursday's announcement didn't specify how many advertisers are involved in the early radio tests nor set a timetable for opening the service to all comers.

Google is betting its technology can do for radio what it has already done for the Internet by automating the process for selling and distributing ads to an audience where the messages are most likely to pique consumer interest. As it does on the Web, Google plans to charge a commission for helping radio stations sell ads.

The Mountain View-based company signaled its intention to expand into radio advertising in January with a $102 million acquisition of dMarc Broadcasting Inc. Since then, Google has been working to make the service compatible with a system that already serves millions of Internet advertisers.

The Internet ad platform has turned into a gold mine, with Google's profit this year expected to approach $3 billion - nearly a 30-fold increase since 2002. The company makes virtually all its money from short, written ads posted on the Web, raising worries among some analysts about Google's lack of other moneymaking channels.

As part of its expansion efforts, Google also is trying to help newspaper and magazine publishers fill some of their unsold advertising space. Google's early efforts in magazines have had little impact. The company just started working with 50 of the nation's largest newspapers.

Google appears intent on pouring far more resources into the radio service, with management openly discussing plans to employ about 1,000 workers in the division.