White House spokesman Scott McClellan has announced his resignation, amid a shake-up of US President George W Bush's senior advisers.
Mr McClellan has been the public face of the White House
"It's going to be hard to replace Scott, but... he made the decision and I accepted it," Mr Bush said.
Mr McClellan has been in the job for nearly three years.
In another move, Mr Bush's chief policy adviser Karl Rove is giving up his policy role to focus on the November mid-term elections.
The policy role will be taken over by Joel Kaplan, the deputy White House budget director.
Speaking alongside the president, Mr McClellan, 38, said he was looking forward to the next chapter of his life.
"I have given it my all sir and I have given you my all sir," he said. "I will continue to do so as we transition to a new press secretary.
WHITE HOUSE APPOINTMENTS
Joshua Bolten, chief of staff
Rob Portman, budget director
Joel Kaplan, deputy chief of staff
Susan Schwab, chief trade negotiator
"The White House is going through a period of transition. Change can be helpful and this is a good time and good position to help bring about change," he added.
Mr Bush said Mr McClellan had had "a challenging assignment", which he had handled with class and integrity.
"One of these days, he and I are going to be rocking in chairs in Texas and talking about the good old days," Mr Bush said.
There was no immediate word from the White House on Mr McClellan's successor.
The resignation comes days after the new White House Chief of Staff, Joshua Bolten, took office.
Many within the Republican party have been demanding a shake-up of the White House staff for several months, says the BBC's Adam Brookes in Washington.
The president's public approval ratings remain in the doldrums, while Iraq and high oil prices hang like dark clouds over the presidency, our correspondent says.
Mr Bush seems unable to get much done domestically either - his push to reform immigration law, this year's big idea, has stalled in Congress.
On Tuesday, former US trade representative Rob Portman took over as the government's budget director at the Office of Management and Budget.
Mr Portman's deputy, Susan Schwab, is to replace him as Washington's chief trade negotiator.