Mr Fujimori denies charges of corruption and humans rights abuses
Peru's ex-intelligence chief Vladimiro Montesinos is to testify in the trial of former President Alberto Fujimori, who is accused of human rights abuses.
The judge summoned Mr Montesinos - who is currently serving a jail term for arms smuggling and is alleged to have organised death squads in the 1990s.
Prosecutors hope he will support their case that Mr Fujimori ordered two massacres that killed 25 people.
The former president, who faces up to 30 years in jail, denies the charges.
Correspondents say nobody knows whether Mr Montesinos will testify in favour or against his former boss, or choose to remain silent.
Mr Montesinos, who is currently serving a 20-year sentence, was Mr Fujimori's intelligence chief during his 1990-2000 rule.
As such, he was privy to the government's policies and strategy in its war against left-wing rebels.
1991 Barrios Altos killings: 15 dead
1992 La Cantuta killings: 10 dead
1992 illegal detention: journalist Gustavo Gorriti and businessman Samuel Dyer
Separate trial on corruption and illegal wiretapping charges
Mr Fujimori's trial relates to two massacres carried out by a death squad known as La Colina, in which a total of 25 people died.
In 1991, La Colina raided a barbecue in a poor suburb of Lima known as Barrios Altos and killed 15 people.
The following year, they kidnapped nine students and a professor.
They were taken away from the campus and summarily executed. Their remains were later found in an unmarked grave.
It is alleged the death squad was under the direct command of the Peruvian president.
Mr Fujimori is also charged with ordering the illegal detention and interrogation of a prominent journalist, Gustavo Gorriti, and businessman Samuel Dyer, in 1992.