Millions of Venezuelans voted for the second time on a proposal to allow socialist President Hugo Chavez to stay in power for as long as he wins elections.
Mr Chavez says he needs another 10 years for his revolution to take root in South America's top oil exporter.
He holds a slim edge in polls but many Venezuelans were expected to decide late whether they supported scrapping term-limits.
Mr Chavez narrowly lost a vote in December 2007 to end term limits and if he loses his second try he would have to leave office in 2013 or find another way to change the rules.
The capital's residents lined up at polling stations before dawn. Voting went ahead with few problems and turnout appeared to be high, authorities said.
Both sides claimed exit polls showed they were winning.
Confident of victory, Mr Chavez says a win will reinforce his mandate to forge a socialist state and challenge Washington in Latin America.
