Centre-left candidate Daniel Scioli is leading exit polls in Argentina's election, but it is not clear if the vote will go into a run-off.
Mr Scioli was handpicked as candidate by two-term leader Cristina Fernandez de Kirchner, who is constitutionally barred from seeking a third term.
The head of the campaign led by his main challenger, centre-right Buenos Aires mayor Mauricio Macri, says the election is heading for a second round.
A run-off would be held on 22 November.
Argentina's C5N television network said Mr Scioli had won by "a large margin".
To win outright, a candidate needs 45% of the vote or a minimum of 40% as well as a 10-point lead over the nearest rival.
Mr Scioli, the governor of Buenos Aires province, is a former world powerboating champion who lost his right arm in a race in 1989.
Last week, he pledged tax cuts for workers earning under a certain income, a move expected to affect half a million people.
He has also vowed to bring down Argentina's inflation to single digits in less than four years and promises to introduce policy changes to invigorate the economy.
Another candidate, Sergio Massa, a former ally of Ms Fernandez, is polling behind Mr Macri. There are three other names on the ballot paper, with 32m people eligible to vote.
Long queues formed outside polling stations from the early hours of Sunday.
Ms Fernandez, who stands down after eight years in power, says she leaves Argentines a better country.
"We are voting today in a completely normal country," said Ms Fernandez said after casting her vote in the Patagonian town of Rio Gallegos.
In previous decades, Argentines always went to the polls "in the middle of a serious crisis," she added.
Ms Fernandez said achieving stability and leaving Argentines "a normal country" was the promise made by her late husband, Neston Kirchner, when he took office in 2003.
He died in 2010, three years after handing over the presidency to his wife.
Whoever wins the presidency faces significant economic challenges.
While the country gained strength after a financial crisis in 2002, its economy, the third-largest in Latin America, has slowed in recent years, with GDP growing by only 0.5% last year.
The government is also locked in a battle against American hedge funds who disagree with how it wants to restructure $100bn (£65bn) of debt on which it defaulted in 2001.
While the firms successfully sued Argentina for repayment, Ms Fernandez refused to pay.