Almost a year ago, Mayor Bloomberg said on his weekly radio show that he would like to facilitate new stadium deals for both the Yankees and their crosstown rivals, the Mets.

“If the Steinbrenner organization comes up with money, we’ll look at that. If it’s right for the city, the city will help them,” he said. “If the Wilpon organization comes up with money for a new Shea Stadium and it’s right for the city, the city will help them. I mean, I’d love to see both.”

This week, he got his wish.

Yesterday, Mr. Bloomberg joined Governor Pataki; the Yankees owner, George Steinbrenner, and other state and local officials to announce the team’s plans for a new, $800 million baseball stadium adjacent to its current home in the South Bronx.

The announcement came just three days after the mayor joined the Mets owner, Fred Wilpon, to announce that the National League franchise plans to build a new, $600 million stadium adjacent to its existing stadium in Willets Point. Both ballparks are scheduled to open for the 2009 season.

The city and the state will pay for the infrastructure costs associated with both stadiums. Planned public expenditures in the Bronx – where the city is to spend $135 million to create new parks and enhance infrastructure and the state is to pay $70 million for construction of additional parking facilities – exceed public expenditures in Queens, where the city will spend $85 million and the state will spend a proposed $75 million to enhance infrastructure and prepare the site. If New York City is selected as host city for the 2012 Olympics, the Queens stadium would become its principal venue and the city, the state, and the NYC2012 organization would finance a temporary expansion of the stadium.

“I guess all you can say is wow – what a week for baseball and what a week for this city,” Mr. Bloomberg said yesterday.

The mayor, who rode the no. 4 train to Yankee Stadium from City Hall for the announcement, said the city has spent $30 million in the past five years for the upkeep of the old stadium and would have to allocate hundreds of millions of dollars more in the coming decades to maintaining the aging facility. The city will not have to finance the upkeep of the new stadium, which will be owned by the baseball team.

“Instead of spending hundreds of millions to maintain the old stadium, this agreement relieves us from that burden and allows us instead to spend that money on parks and infrastructure that the public will use for decades to come,” Mr. Bloomberg said, estimating that the city’s $135 million investment would produce $350 million in new revenue and savings in the next 30 years.

Mr. Pataki, who reminisced about his first experience at Yankee Stadium as a boy and about his fond memories of taking his own son there, said the Yankees deal would create more than 3,600 construction jobs.

“It’s a great day,” he said. “It’s a day we can be proud of.”

The new stadium, which was designed by HOK Sport + Venue + Event, is the same firm that did preliminary design for the Mets’ stadium. It will have between 50,800 and 54,000 seats – somewhat fewer than the current Yankee Stadium, but more than the planned Mets home.

The Yankees president, Randy Levine, said the design of the new stadium was inspired by the design of the original stadium, which opened in 1923. He said the wide concourses in the new stadium would ensure that fans have top-notch views, even when they are at the concession stands, which will be more plentiful than in the current stadium.

“You’ll never miss an inning, you’ll never miss a pitch,” Mr. Levine said as he presented large, color renderings of the new design. “You’ll be able to see the playing field wherever you are.”

The president of the Partnership for New York City, Kathryn Wylde, said the construction of two new ballparks in the same four-year period would prove to be positive for the Bronx, for Queens, and for the city as a whole.

“These are both stadium projects that represent important centers of economic activity in Queens and in the Bronx,” she said. “The fact that they’ve waited patiently on queue during all the attention to the Jets, I think it’s appropriate that this is all coming together at the same time.”

She said both stadium deals had been “90% cooked” for quite some time, but she said it seems that the apparent demise last week of the proposed New York Sports and Convention Center on the far West Side of Manhattan, where the Jets would have moved, triggered the completion of plans for both projects.

“Sometimes progress is precipitated by a third-party action,” she said. “In this case, I think that’s what happened. It was a great opportunity to kind of move both these projects forward. Out of a disappointment or a failure, the mayor has plucked a real positive for the boroughs of the Bronx and Queens.”

Both the Yankees project and the Mets project need approval from state and city lawmakers to progress. The speaker of the state Assembly, Sheldon Silver, and the Senate majority leader, Joseph Bruno, who blocked the Jets stadium last week, have already committed to the Mets’ proposal. Yesterday, leaders including the City Council speaker, Gifford Miller; Assemblymen Jose Rivera and Carmen Arroyo; and Bronx Borough President Adolfo Carrion Jr. endorsed the project and said they didn’t expect it to run into problems in the approval process.

Referring to the Nets, Mets, and Yankees deals, Ms. Wylde said, “My crystal ball says that all three projects will receive overwhelming support from all approving public bodies.”

Mr. Steinbrenner, the longtime Yankees owner, called the future promising.

“No, I don’t think so,” he said when asked if approval would be a problem. “So far, everyone has been on board.”

Yankee Stadium opened in April 1923 and was renovated in the mid-1970s. In 1993, the state unveiled a plan to build a $319 million stadium for the team on the far West Side, at the same site the Jets have been chasing in recent years. That plan failed. In the Giuliani years, the Yankees threatened to move to New Jersey, and the mayor, an ardent fan of the team, offered them a new home on the West Side to keep them in the city. When that plan fell through because of community and political opposition, Mr. Giuliani brokered a deal for an $800 million Bronx stadium with a retractable roof.