Polls opened this morning in a closely fought by-election in the Prime Minister's backyard.
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.
Polls opened at 7am and will close at 10pm.
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 yesterday, 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.
Labour candidate Lindsay Roy is headteacher of Mr Brown's former school and will cast his vote later this morning at a local primary school.
Peter Grant, who is standing for the Nationalists, is the leader of the local Fife council.
Lib Dem candidate Harry Wills has campaigned for a 2p cut in income tax to help hard-pressed families, while Tory Maurice Golden says a vote for him will send a "message of change" to Mr Brown.
The most recent Parliamentary by-election in Fife saw the Liberal Democrats capture Dunfermline and West Fife in 2006 - overturning a Labour majority of more than 11,000.
The Glenrothes constituency neighbours Mr Brown's own Kirkcaldy and Cowdenbeath seat.
The by-election is taking place following the death of Labour MP John MacDougall.
The SNP's Peter Grant voted early with his wife Fiona.
After casting his vote at the Collydean Primary School, he said: "The signs are we are going to win.
"There's still a lot to be done.
"We have a fantastic team of volunteers out now making sure everyone who has promised us their vote will deliver that.
"If they do, as I think they will do, we will be the ones that are celebrating this evening."
Forty minutes later Labour's Lindsay Roy arrived at the same polling station to cast his vote with his wife, Irene.
He said as he left: "I am confident, optimistic, but not complacent.
"We need to get our vote out. I think we have a very good chance of winning.
"I think eventually we will be at the front when the poll is counted."Reuse content