Janet Yellen has been sworn in as chair of the US central bank, the Federal Reserve.
At the same time Wall Street closed at its lowest level since last June.
Her appointment comes at a time when the US economy appears to be improving.
She had already taken up her functions since being nominated by President Obama in October and confirmed in her post by the Senate last month.
There will be an immediate test this week when new employment figures are published this Friday.
Janet Yellen’s term begins as the Fed sets about winding down its huge economic stimulus programme.
As vice chair to her predecessor, Ben Bernanke, she was a staunch supporter of the scheme.
Soon she will appear before a congressional committee to explain her views on the economy and regulation.
Janet Yellen has also been warned that time is short to resolve the latest budget battle.
A new deal is expected to be struck this week.