ABC announced Thursday that the former "Today" show anchor will be guest host next week on "Good Morning America," the rival wake-up show that has been rising in the ratings.
She will sub for the vacationing Robin Roberts for a week, teaming with George Stephanopoulos.
Couric was co-host of "Today" in December 1995, when the NBC show's streak began. "Today" has won every week in the ratings since then, 850 consecutive and counting, according to the Nielsen company.
Yet frisky "GMA" has been making noise lately. Last week the ABC show was only 137,000 viewers behind "Today" (an average of 4.98 million to 4.84 million), the closest the two shows have been since 2008.
"This has been one of the longest marathons of all time," said Tom Cibrowski, senior executive producer in charge of "Good Morning America." ''There will eventually be a time when the No. 1 spot turns over. We strive every day to take over the No. 1 position. We never stop working on that."
He's not making any predictions about next week, but the curiosity factor of Couric in a new morning chair seems sure to pull in some viewers.
Couric was co-host of NBC's "Today" for 15 years before leaving the network in 2006. Before jumping to ABC last year, she was at CBS, where she anchored the "Evening News."
The winning streak has a big psychological impact in one of the most important parts of the day on television for the broadcast networks. Morning shows are hugely profitable at a time of declining viewership, and none has been more of a cash cow than "Today."
The closest "GMA" got to breaking the streak, in the spring of 2005, NBC fired the "Today" show executive producer and installed the current boss, Jim Bell.
"You kind of wait for these times in morning television, when you get a team together that clicks," Cibrowski said. "We have a team that is on fire. We have the big 'C.' We have the chemistry now."
NBC had no comment on Thursday. Privately, some at NBC suggest that ABC's stronger prime-time lineup is helping "GMA," particularly when the morning show takes advantage of it by featuring stories on "Dancing With the Stars," for example. Last week, ABC was boosted by having actors from the hottest movie, "The Hunger Games," on the show each day of the week.
ABC's best chance of ending the streak would likely come within the next two months. NBC televises the Olympic games from London this summer, and the Olympics traditionally give a boost to "Today."
"Today" is also awaiting a decision by its top anchor, Matt Lauer, on whether he wants to continue in the morning.