An inmate at the Don Jail was rushed to hospital Wednesday after he was found with no vital signs and signs of trauma.
Paramedics managed to resuscitate the man before rushing him to a local trauma centre in Toronto but emergency crews said the man was in critical condition.
A spokesperson with Emergency Medical Services said the nature of the man's injuries are unknown and that it is unclear if the man was attacked or suffered a medical emergency.
The inmate, who is believed to be in his 30s, was found on the fifth floor at around 12:30 p.m.
The jail, located on Gerrard Street, is a holding facility for short-term prisoners who are generally either awaiting trial or sentencing.
This past weekend, a 32-year-old inmate at the Don Jail was beaten to death after he got into an squabble, allegedly over a bag of potato chips.
Jeff Munro was attacked in an area of the jail where there are no surveillance cameras.
Police arrested three people in connection with the case and have charged them with first-degree murder.
