Voters are going to the polls in a closely fought by-election on which much of Gordon Brown's political reputation is riding.
Labour holds a majority of more than 10,000 at Glenrothes - but the SNP overturned a bigger deficit to win Glasgow East earlier this summer.
A Labour failure to hold the seat next door to Mr Brown's Kirkcaldy and Cowdenbeath constituency would raise fresh questions about his leadership, although the world's economic woes have to some extent overshadowed the contest.
Mr Brown has twice travelled to the constituency to campaign there, making it the first by-election he has campaigned in since becoming Prime Minister.
His wife Sarah has also campaigned at the seat.
Former Deputy Prime Minister John Prescott was on the campaign trail on Wednesday, while Chancellor Alistair Darling has also visited the constituency, as has Tory leader David Cameron and Nick Clegg, the Lib Dem chief.
But Scotland's First Minister Alex Salmond, the SNP leader, has visited the constituency more than 10 times and is confident his party can pull off another shock.
The by-election is taking place following the death of Labour MP John MacDougall.
Eight candidates are contesting the November 6 by-election:
Morag Balfour (Scottish Socialist Party); Maurice Golden (Scottish Conservative and Unionist); Peter Grant (Scottish National Party); Louise McLeary (Solidarity - Tommy Sheridan); Jim Parker (Scottish Senior Citizens' Unity Party); Lindsay Roy (Scottish Labour Party); Kris Seunarine (Ukip); Harry Wills (Scottish Liberal Democrats).
