Gen Mejia, 80, is accused of ordering massacres in indigenous Maya villages during Guatemala's 1960-96 civil war.
His lawyers say he recently suffered a stroke and is not physically or mentally capable of answering the charges.
Prosecutors dispute this, saying he was under sedation at an initial hearing.
Oscar Mejia was arrested in Guatemala City earlier this month after a warrant was issued against him on charges of genocide.
He ruled Guatemala from 1983-86 after seizing power in a coup.
He also served as an army general and defence minister during the long civil war, in which an estimated 200,000 people were killed as the army battled left-wing guerrillas.
A UN-backed truth commission found that the security forces were responsible for the vast majority of killings.
It also concluded that massacres of indigenous Maya communities accused of supporting the rebels amounted to acts of genocide.
Few senior army commanders have ever faced trial for the atrocities, but in recent months the government of President Alvaro Colom has begun to try to bring suspected war criminals to justice.
Another former general - Otto Perez Molina - is favourite to win the presidential election next month.
President Colom is not allowed to stand for a second term.
Former Guatemalan military ruler Oscar Mejia Victores has been sent to hospital to see if he is well enough to stand trial on genocide charges.