Autonomy supporters celebrated after exit polls were released
Bolivia's gas-rich Tarija province has voted overwhelmingly in favour of greater autonomy, exit polls suggest.
About 80% of the voters backed the measure in a referendum, several pollsters said.
The result is being seen as a rejection of President Evo Morales' drive to redistribute wealth in South America's poorest nation.
Tarija is the fourth province to back greater autonomy. The central government says the polls are illegal.
According to several exit polls, just over 80% of those voting in Tarija voted Yes in the referendum.
However, pollsters say that most supporters of President Morales did not vote.
Some voters were deterred by unusually cold weather and a few minor outbreaks of violence, the BBC's Daniel Schweimler says.
Electoral officials have said final results are expected later this week.
Autonomy backers celebrated in Tarija's capital after the exit polls were released.
"A new Bolivia must be built on a foundation of autonomy. Centralism has left a bad legacy," regional Governor Mario Cossio told a crowd of supporters late on Sunday.
Earlier this year, the provinces of Santa Cruz, Beni and Pando also voted to distance themselves from the central government in La Paz.
Tarija's referendum is another brick in the wall that is increasingly dividing the poorer, predominantly indigenous western half of Bolivia from the gas and oil-rich eastern half, our correspondent says.
President Morales, who has more than two years left in office, faces a recall referendum on his leadership in August.
The referendum will test whether Mr Morales still has the people's support
If successful in the ballot, Mr Morales says he wants to hold a public referendum on a draft constitution which has been awaiting approval since last year.
The constitution aims to enshrine reforms such as land redistribution to Bolivia's indigenous majority and sharing of wealth with the poorer western regions.
However, critics say it cedes too much control to the government in La Paz.
Mr Morales's opponents in the eastern states argue that his plans would unfairly privilege indigenous groups and would mean greater central control.
The proposals also include allowing the president to stand for re-election for another five-year term.