Conservative leader Mauricio Macri has won Argentina's presidential runoff vote, according to exit polls.
The win heralds an end to 12 years of leftist government.
Exit polls broadcast by several Argentine television channels including TN, C5N and Canal 26 just after voting closed indicated Mr Macri, 58, had won the vote, without giving a breakdown.
Official results are due in the coming hours, but cheering, dancing crowds of Mr Macri's supporters were already celebrating at his post-election rally in a conference centre on the outskirts of Buenos Aires.
His rival Daniel Scioli's camp, gathered in a city centre hotel, had yet to react.
Mr Macri climbed in the polls after losing closely in the first round vote on 25 October to Mr Scioli, an ally of the combative outgoing president Cristina Fernandez de Kirchner.
That result forced Argentina's first ever presidential runoff.
If today's result is confirmed, Mr Macri will become the most economically liberal president since the 1990s at the helm of Latin America's third-biggest economy.
He has capitalised on discontent among voters who said they were fed up after 12 years of rule by Ms Kirchner and her predecessor and late husband, Nestor Kirchner.
With social welfare programmes and protectionist economic policies, they won affection among poorer Argentines and mistrust among foreign investors.
They presided over an economic recovery that now appears to be flagging, with slowing growth and soaring inflation.
Mr Macri has vowed to free up trade, liberalise the economy and negotiate with Argentina's foreign debtors.