Up to 30,000 people disappeared or were killed in the Dirty War
Relatives of the thousands of people who were tortured and killed during military rule in Argentina are watching Ricardo Cavallo's case very closely.
He is being questioned by the campaigning Spanish judge, Baltasar Garzon, who has long fought to bring those suspected of human rights abuses in Latin America to justice.
Mr Garzon was also behind the unsuccessful moves to try the former Chilean leader, Augusto Pinochet.
Ricardo Cavallo left Mexico on Saturday under a heavy armed guard, with Argentine protesters calling him a murderer and banging on the roof of the police car taking him to the airport.
Human rights activists said this is the first time one country has extradited a person to another country to stand trial for human rights crimes allegedly committed in a third.
The former army officer was arrested in Mexico three years ago, after a Mexican newspaper accused him of being a former intelligence officer.
They said he worked for the notorious School of Naval Mechanics in the Argentine capital, Buenos Aires, which served as a secret torture centre.
He denies the charges, but would probably not have stood trial in Argentina thanks to an amnesty for those who served during the military government, in power from 1976 to 1983.
Up to 30,000 people were killed or disappeared in the Argentine military's campaign in the 1970s against what it called left-wing insurgents, in what was called the Dirty War.
Many were tortured, drugged and thrown from aircraft into rivers and the sea.