Anders Breivik has allegedly confessed to the massacre on July 22. He allegedly killed eight people with a truck bomb in Olso, and then fatally shot 69 people at a nearby youth camp.
Breivik said the attacks were an attempt to purge Europe of Muslims. He said in a 1,500-page manifesto posted online before the attacks that they are an attempt at cultural revolution.
Breivik arrived at the closed hearing in a dark car, dressed in a dark suit, under heavy police protection.
The court decided he should stay in isolation for fear he could tamper with evidence or contact possible accomplices. The court says they are still unsure if he acted alone.
Judge Hugo Abelseth acknowledged Breivik had described his isolation as "boring and monotonous, and a sadistic torture method," but said he must nonetheless spend at least four more weeks there. He said the next hearing would be held on Sept. 19.
Breivik appeared calm at the hearing but showed no remorse, his lawyer Geir Lippestad said.
The Associated Press contributed to this report.