Signaling a possible shake-up among President Bush's senior advisers, the new White House chief of staff told top presidential aides Monday to expect changes that "refresh and re-energize the team." He invited anyone who is thinking of leaving before year's end to do so now.

Joshua Bolten, who took over the top staff job late Friday, did not ask for anyone's resignation in his first Monday morning staff meeting with the president's senior aides, presidential spokesman Scott McClellan said. No one stepped forward to say they would leave, either, McClellan said.

But Bolten has Bush's full authority to make changes to the president's staff, which has had a low turnover rate, with many aides serving him for years. Bolten already has had closed door meetings with some top advisers and plans more in the coming days, McClellan said.

McClellan said Bolten told the aides to expect "some changes and adjustments" after he's gone through the process of talking to the staff. At the same time, McClellan said that "you have to balance change with continuity."

He quoted Bolten as saying he was assuming his new job at a challenging time when the United States was engaged in a war on terrorism.

"And Josh talked about how this is a time to refresh and re-energize the team and for all of us to renew our commitment as we go forward," McClellan said. "And he talked about how we have an active and full agenda."

Bolten is only Bush's second chief of staff. His predecessor, Andy Card, served Bush for more than five years but resigned amid tensions with Congress, all-time low approval ratings for Bush and calls from Democrats and Republicans to bring in a fresh perspective.

Bolten had served as the president's budget director and will be announcing a replacement for that position soon, McClellan said. The job of domestic policy adviser at the White House is open, as well. But further changes are clearly on the horizon, and Bolten gave top aides the option of leaving first.

"He wanted to make sure he had the team in place that is going to be here for a minimum of the remainder of the year," McClellan said. "And he said if people are thinking about leaving, that now is the time to come to such a decision."

McClellan would not comment on whether he plans to continue working at the White House.

"I never speculate about personnel measures," McClellan said, repeating his standard reply to questions about staff changes with a smile.