Foster and McGuinness write to Theresa May laying out their Brexit priorities.
The first and deputy first ministers have demanded they must be fully involved in negotiations with regards the UK's future relationships with the EU and other countries.
Arlene Foster and Martin McGuinness have written an open letter to Prime Minister Theresa May laying out their concerns.
The First and Deputy First Ministers - who were on opposite sides of the EU debate - raised issues including the border, energy and EU funds which they said were of "particular significance" for Northern Ireland.
The letter read: "We therefore need to retain as far as possible the ease with which we currently trade with EU member states and... there is also the matter of the many thousands of people who commute each way across the border to work on a daily basis."
It also addressed the importance of EU funds to the north and concerns over the potential of tariffs on the agri-food sector.
"Since 1994, for example, we have benefited to the tune of €13 billion from Europe... the current uncertainty around the ability to draw down a proportion of these funds, and the absence of EU programmes in the future is of real concern to a range of sectors."
Although 52% of the UK as a whole voted to leave the EU in June's referendum, a majority in Northern Ireland, 56%, backed the Remain campaign.
Ulster Unionist MLA Steve Aiken asked whether the letter represented "a DUP u-turn" on its pro-Brexit position.
Mrs Foster said "Brexit means Brexit, but that doesn't mean that we close our eyes to the challenges that are there."
Claiming the UUP position on the referendum had been "comical", she added: "Poor Steven Aiken has been sent out once again to be the attack dog against the Executive and frankly, he comes across more as a Chihuahua."
Mr Aiken retorted on Twitter that it was the first time he had "ever been compared to small but intelligent and ferocious Mexican attack dog".
- first time I've ever been compared to small but intelligent & ferocious Mexican attack dog!! @uuponline— Steve Aiken OBE (@SteveAikenUUP) 11 August 2016
SDLP South Down MP Margaret Ritchie also expressed amazement at the letter.
"The full letter makes as convincing as case for staying in the EU as any heard before the referendum.
"I suspect the heart of the issue is that the DUP leadership never seriously thought that Britain would vote to Leave, and they are now scrambling to deal with the consequences."
Deputy First Minister Martin McGuinness said it was important he and Mrs Foster "come together in the aftermath of the vote" in spite of having been on different sides of the referendum debate.
"This is about how we protect the interests of the people we represent and the challenges that lie ahead."
"...This is too big, too serious for us not to be joined up in relation to how we take this process forward."
Mrs Foster defended her part in the letter and insisted there were "huge opportunities" associated with Brexit.
She told the BBC: "It would be negligent of me not to point out where I believe the challenges are.
"We're extracting ourselves from European Union and it is of course right that we identify where those challenges lie, but I fundamentally believe that there are huge opportunities."