US President Donald Trump's nominee has been sworn in to lead the Environmental Protection Agency.
Scott Pruitt, a fierce critic of the agency, was approved by the Republican-controlled Senate despite Democratic attempts to delay the confirmation.
A judge in Oklahoma on Thursday ordered him to release, by Tuesday, emails he exchanged with oil and gas executives.
Democrats boycotted an earlier vote over his email refusal and claimed he was too close to energy companies.
They held the Senate floor through the night to put pressure on the chamber to delay the confirmation vote until the emails, which were exchanged while he was Oklahoma attorney general, were handed over.
But the Senate voted in favour of his confirmation by 52 to 46, and he was sworn in on Friday.
Mr Pruitt, 48, had refused to hand over the emails, at the request of a liberal watchdog, for more than two years.
Oil and gas industry stooge? Analysis by Matt McGrath, BBC environment correspondent
Mr Pruitt is perhaps the most controversial appointment in the history of the EPA - the Oklahoma attorney general has spent years fighting the role and reach of the organisation he now heads.
Hundreds of former EPA staff members wrote an open letter against his appointment, some calling him an "unqualified extremist".
Environmental campaigners see him as an oil and gas industry stooge who is "lukewarm" on the threat posed by climate change - they fear that hard-won environmental regulations will be overturned.
Key among them is the 2009 ruling that greenhouse gas emissions endanger both the environment and public health. This underpinned many of the actions taken by President Obama to curb CO2.
With Mr Pruitt in place, it's likely that President Trump will rapidly push ahead with orders to overturn Obama's Clean Power Plan and his "Waters of the US" rule, something conservatives also see as an over-extension of federal power.
Mr Pruitt will probably attempt to cut through the "regulatory rampage" that Republicans believe the EPA has embarked on over the past eight years - but he needs to be wary of over-reach.
President Reagan appointed Anne Gorsuch Burford to reform the EPA back in 1981 - but after failed attempts to downsize, she was out on her ear two years later.
Republicans predicted Mr Pruitt would reset the agency's goals.
"I have no doubt the Scott will return the EPA to its core objectives," said Oklahoma Sen James Inhofe, accusing the agency of "federal overreach, unlawful rule making and duplicative red tape".
"EPA has made life hard for families all across America," said Wyoming Sen John Barrasso, adding that Mr Pruitt would "bring much needed change".
Environmental groups said they feared he would loosen regulations on energy companies.
During his confirmation hearing he had said he disagreed with Mr Trump saying climate change was a hoax but he previously cast doubt on the overwhelming evidence that changes in the earth's temperature were down to humans.
"Scott Pruitt is the worst pick ever confirmed to lead the EPA," said Rhea Suh, president of the Natural Resources Defense Council.
"We'll use every tool in the kit to stop him from harming our air and water, endangering our communities and surrendering our kids to climate catastrophe."
Mr Pruitt's confirmation means that President Trump has filled 14 of his 22 cabinet posts.
He has blamed Democratic obstructionism for not filling more of his key jobs by now.