Bulgarian President Georgi Parvanov will visit Libya later this week in an attempt to save five nurses accused of infecting children with Aids.
Col Gaddafi has so far shown an inflexible stance on the nurses' fate
He was officially invited by Libyan leader Muammar Gaddafi and is expected to discuss the fate of the medics.
The five Bulgarians and one Palestinian doctor are currently on death row, and Libya's supreme court is due to rule on their appeal on 31 May.
But the six, detained since 1999, claim they were tortured into confessing.
They were convicted by a Libyan court in 2004 of knowingly infecting more than 380 Libyan children with tainted blood as part of an experiment.
Forty-seven of them have died since.
Experts, including Luc Montagnier, a co-discoverer of the virus, say the Aids epidemics broke out due to poor hygiene at the Benghazi hospital where the incident took place.
They also say that the infection spread before the nurses' arrival at the hospital.
The US, the UN and the European Union have firmly condemned the verdicts, but Col Gaddafi has so far refused to release the nurses, who are due to be executed by firing squad.
His demand for a compensation equal to that paid by Libya to the families of the 1988 Lockerbie bombing has been rejected by Bulgaria on the grounds that it would be tantamount to acknowledging the nurses' guilt.
Mr Parvanov's visit, which begins on Thursday, is the first of a Bulgarian leader to Libya since the end of the communist era.