SNP leader Alex Salmond said independence was "not a one-way street" and a Scotland which went its own way could later re-join the United Kingdom.
Alex Salmond said independence was not a one-way street
His comments came as an opinion poll suggested the SNP were on course to become the biggest party at Holyrood.
The YouGov poll put the SNP on 38% in the constituency vote, eight points ahead of Labour and on 32% in the list vote, five points ahead of Labour.
Some 1,872 people were polled between 17 and 23 April.
It was the biggest sample of the campaign.
According to the poll, the Liberal Democrats were running at 15% in the constituency vote and 12% in the regional list.
The Scottish Tories were at 12% and 14%.
The YouGov poll was funded by the Economic and Social Research Council and carried out for the universities of Strathclyde, Lancaster and Sheffield.
Support for independence is put at 32% in the poll.
A Labour spokesman said the poll underlined the "real choice" faced by voters in Thursday's elections.
"If people want a strong and stable economy, with more jobs and investment for schools and hospitals they should vote Labour, or the SNP alternative of high tax, turmoil, instability, cost and risk," he said.
When asked if an independent Scotland could in the future go back to being part of the UK, Mr Salmond told the GMTV Sunday Programme: "Any nation can do whatever it pleases.
"It can vote to become independent and it can vote if it so chooses to become un-independent, that's the prerogative of a nation and Scotland, of course, is a nation.
"It's a country - it has the right of self-determination. So no, it's not a one-way street, theoretically a country could do that."
Jack McConnell was campaigning in Edinburgh on Sunday
On the campaign trail, the Liberal Democrats claimed to be the only party in the election committed to turning Scotland into Europe's "powerhouse" for green energy.
Party leader Nicol Stephen said their key green policies were 100% renewable electricity by 2050, 40,000 energy friendly homes with wind turbines and solar panels, £300 tax rebates and grants for micro-renewables and energy efficiency, and a 70% target for recycling.
Campaigning in Aberdeen, Mr Stephen said: "I am determined to see wave machines generating electricity in Scottish waters by the end of this year.
"There is a real choice at this election.
"That choice is between new nuclear power in Scotland or a clean, green future for families and future generations."
Tory leader Annabel Goldie said prisoners caught with drugs would have their visiting rights suspended.
The move would form part of her party's "complete overhaul" of Scotland's prison regime, she said.
Ms Goldie said Scots were rightly "outraged" by the feeling that prisoners were being pandered to areas ranging from compensation payments for slopping out, to prisoner demands for the right to vote.
"Some politicians don't want to tackle difficult issues like this," she said.
"I'm not one of them."
Scotland's Labour leader Jack McConnell was on the streets of Edinburgh on Sunday appealing to undecided voters.
During his tour of shopping centres, he urged the electorate to back Labour rather than "gamble" with the SNP.