At a press conference in Oslo Friday, Judge Wenche Arntzen appointed psychiatrists Agnar Aspass and Terje Toerrissen to perform a second evaluation of Breivik.
The decision comes after prison psychologists monitoring Breivik found him showing no signs of mental illness last month.
Breivik, 32, confessed to opening fire on a youth summer camp, killing 69, and setting off a bomb in Oslo's government district, killing eight on July 22. He released a manifesto the day of the attacks that claimed the killings were necessary to alert Norwegians to the dangers of Islam.
In November, forensic psychiatrists diagnosed him with paranoid schizophrenia, a conclusion widely criticized by other mental health professionals who argue a paranoid schizophrenic could not undertake such a premeditated plot.
Aspass and Toerrissen have been appointed to deliver a second opinion, The Daily Telegraph reports.