Polling stations for some 1.6 million eligible voters in the small but prosperous Alpine country will close at 2230 IST, with first official results due three hours later.
The winner will be inaugurated only days before Slovenia takes over the 6-month rotating presidency of the EU on January 1.
Although a largely ceremonial figure, the president will feature prominently in international contacts during the presidency.
President Janez Drnovsek, a left-winger who has frequently clashed with Prime Minister Janez Jansa's conservative government, is not running for a second five-year term.
Although turnout in the first four hours was below 17 per cent, many Slovenians felt the election mattered.
''The presidential function is important and we need someone who will connect with the nation. Drnovsek recently became very passive and that is not what we need,'' Tina Brecko, a 27-year old student, said as she cast her vote.
Since 1991, Slovenia's presidents have been left-leaning. A victory for Lojze Peterle, a conservative former prime minister and the leading presidential candidate, would break the pattern.
Slovenia's governments were also mostly centre-left until Jansa's conservative coalition won a parliamentary election in 2004, the year Slovenia joined the EU and NATO. Most parties in the coalition support Peterle.
''Although the president does not have much power, this election is important because it will be an indicator of the political mood in the country ahead of the parliamentary poll (late) next year,'' Borut Hocevar, an editor at daily Zurnal 24, told Reuters.
Peterle, 59, is likely to win the most support of the seven candidates -- between 26 and 41 per cent -- but probably not enough to secure outright victory in the first round.
''Opinion polls showed that it is practically impossible for Peterle to get more than 50 per cent in the first round and I expect a very tight race in the second round,'' Hocevar said.
In the second round, due on November 11, Peterle would be likely to face off against Danilo Turk, a former diplomat, or Mitja Gaspari, a former central bank governor. Both are supported by left-of-centre parties.
Peterle, a member of the conservative New Slovenia party, was Slovenia's first prime minister, from 1990 to 1992, and is now a member of the European parliament.
In an interview with Reuters this week, he said EU unity would be of crucial importance during Slovenia's presidency of the union, particularly on the question of the future status of the UN-run Serbian province of Kosovo.
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