Spain borrowed 4.799 billion euros ($6.25 billion) in bond auctions on Thursday, paying sharply reduced interest rates for the critical long maturity of 10 years.
Spain, fighting to avoid having to seek rescue funding, placed its 10-year bonds at an average rate of 5.666 percent, down from 6.647 percent at the last such auction on August 2.
A rate of below 6.0 percent is considered important for Spain, since borrowing rates above this level are seen as being unsustainably expensive.
Borrowing rates for eurozone countries in trouble have fallen sharply on the market for existing government debt since the European Central Bank said on September 6 that it would buy unlimited quantities of eurozone debt.
But the ECB said that such help would depend on the countries concerned appealing for rescue funding and accepting consequent conditions for structural reforms, which Spain is reluctant to do.
Spain set out on Thursday to issue three-year and 10-year bonds to raise 3.5-4.5 billion euros.
Total demand for the bonds amounted to 8.6 billion euros.
The debt managers chose to raise 3.940 billion euros with the three-year issue and 859 million euros with the 10-year issue.
The average rate for the three-year maturity was 3.845 percent. At the last similar issue on September 6, the rate was 3.676 percent.
The issue of bonds with a life of 10 years is seen as a critical test.
Countries fund their debt and investment programmes mainly with long-term debt, rather than with more expensive short-term debt which has to be rolled over frequently.
The risk of lending for 10 years is, in normal circumstances, higher than the risk of lending for shorter maturities such as three years, and in the eurozone the benchmark bond by which other bonds and yields are measured is the 10-year German bund.
The situation of the Spanish debt market is watched closely by investors since Spain has the fourth-biggest economy in the eurozone.
Since June, Spain has been promised 100 billion euros of help to refinance its banking system which has been under severe strain since a property bubble collapsed in 2008.