Venezuelans are voting in a referendum on whether to remove the limit on the number of times the president and other politicians are allowed to stand for office.
A "yes" vote in Sunday's referendum would allow President Hugo Chavez to stay in office after his current term ends in 2012.
Mr Chavez has said the constitutional amendment is needed for the future of Venezuela's socialist revolution.
But critics say it is designed to concentrate power in the president's hands for decades to come.
A proposal to end presidential term limits was one of a package of 69 constitutional changes narrowly rejected in a 2007 referendum.