CHICAGO — Oprah Winfrey is announcing today that her powerhouse daytime television show, the foundation of a multibillion-dollar media empire, will end its run in 2011 after 25 seasons on the air.

Winfrey was to announce the final date for “The Oprah Winfrey Show” during a live broadcast, according to her production company, Harpo Productions Inc.

Once a local Chicago morning program, the production evolved into television's top-rated talk show for more than two decades, airing in 145 countries and watched by an estimated 42 million viewers a week in the U.S. alone.

A Harpo spokeswoman declined to comment Thursday on Winfrey's future plans except to say that “The Oprah Winfrey Show,” which has seen ratings slip 7 percent from a year ago, will not move to cable television.

Winfrey, 55, is widely expected to start up a new talk show on OWN: The Oprah Winfrey Network, a much-delayed joint venture with Discovery Communications Inc. that is expected to debut in 2011. OWN is to replace the Discovery Health Channel and will debut in some 74 million homes. An OWN spokeswoman declined comment Thursday.

CBS Television Distribution, which distributes “The Oprah Winfrey Show” to more than 200 U.S. markets, held out hope it could continue doing business with Winfrey, perhaps producing a new show.

“We know that anything she turns her hand to will be a great success,” the unit of CBS Corp. said in a statement. “We look forward to working with her for the next several years, and hopefully afterward as well.”

Winfrey's 24th season opened with a bang, as she drew more than 20,000 fans to Chicago's Magnificent Mile for a block party with the Black Eyed Peas. She followed with a series of blockbuster interviews - Mike Tyson and Evander Holyfield, exclusives with singer Whitney Houston and ESPN's Erin Andrews, and just this week, former Alaska governor, GOP vice presidential candidate Sarah Palin.

As a newcomer, “The Oprah Winfrey Show” chipped away at talk king Phil Donahue's dominance. Later, it turned to inspiration. The show ranged from interviews with the world's celebrities to an honest discussion about Winfrey's weight struggles.

The show also became a launching pad for Oprah's Book Club, which then unleashed best-sellers, from “Song of Solomon” and “Paradise” by Toni Morrison to Wally Lamb's “She's Come Undone” and Elie Wiesel's “Night.”

The loss of “The Oprah Winfrey Show” would be a blow to CBS Corp., which earns a percentage of hefty licensing fees from TV stations that use it - largely ABC affiliates.

“Oprah's been a force of media and there's really no person you can look to out there who you could say, ‘That's the heir apparent,'” said Larry Gerbrandt, an analyst for Media Valuation Partners in Los Angeles.

Winfrey started her broadcasting career in Nashville, Tenn., and Baltimore, Md., before relocating to Chicago in 1984 to host WLS-TV's morning talk show “A.M. Chicago” - which became “The Oprah Winfrey Show” one year later. She set up Harpo the next year, and her show went into syndication.

Winfrey built a media empire. Harpo Studios produces shows hosted by Dr. Phil McGraw and celebrity chef Rachael Ray. O, The Oprah Magazine was the nation's seventh-most-popular magazine this year.

Earlier this year, Forbes scored Winfrey's net worth at $2.7 billion.