Portugal will go to the markets for up to 1.25bn euros (£1.04bn; $1.62bn) with a sale of four and 10-year bonds.
The auction will be closely watched as an indication of investor confidence, as markets see how easily - or not - the debt-hit nation can raise funds.
Yields, or the interest rate Portugal must pay to borrow funds, hit a recent fresh high on its 10-year bonds of 7.3%, before falling to 6.9%.
The country is now likely to offer very high yields to sell its debt.
However, bond buying by the European Central Bank (ECB) should stop yields rising so much that Portugal is forced to seek an EU bail-out.
Portugal is to offer a total of between 750m euros ($972.1m) and 1.25bn euros in four- and 10-year bonds.
"The market will take it down primarily because of what the ECB is doing," said Peter Chatwell, rate strategist at Credit Agricole in London.
"The ECB appears to be proactive enough and we've seen tightening in Portuguese yields so far this week."
'Domestic demand'There has been speculation Portugal could join Greece and the Irish Republic in needing an international bail-out, something it has denied.
The country's borrowing costs have surged as investors worried over its financial health.
However, analysts are not worried about demand at the auction, particularly after the ECB buying.
"I expect the auction to be already sold, with domestic demand enough to cover supply, but it doesn't mean that concerns will go away," said BNP Paribas analyst Ioannis Sokos.
Lisbon has argued its situation is different from Greece and the Republic of Ireland - both of which have agreed to bail-outs from the European Union and International Monetary Fund.
It says that its deficit and debt are lower than those nations, that it has not suffered a bubble in property prices and that its banks are sound.
And the European Commission has said there are no discussions under way on an EU-International Monetary Fund bail-out of Portugal.
Spanish auctionAnalysts believe that while Europe could support Portugal, a bail-out of Spain would stretch the existing bailout fund.
Greece was the first eurozone nation to take a bail-out when a three-year 110bn-euro deal was agreed.
The Irish Republic's 85bn-euro bail-out package was agreed last month.
A debt sale due on Thursday by Spain will also be closely watched by investors.
source : bbc.co.uk
