Anders Behring Breivik attacked a youth camp organised by the governing Labour party on the island of Utoeya, after setting off a car bomb in the capital. He told the court he “acknowledged” the acts committed, but said he did not accept criminal responsibility.
The prosecution earlier gave a detailed account of how each person was killed.
If the court decides he is criminally insane, he will be committed to psychiatric care; if he is judged to be mentally stable, he will be jailed.
In the latter case, he faces a sentence of 21 years, which could be extended to keep him behind bars for the rest of his life.
The 33-year-old Norwegian was found insane in one examination, while a second assessment made public last week found him mentally competent.