A medical condition caused a 26-year-old man to veer off Anthony Henday Drive and into a pond Thursday afternoon, Edmonton police said Friday.
"It appears he had a medical incident," said spokesperson Scott Pattison.
Several people, including an off-duty police officer, spotted the car as it entered the pond becoming fully submerged, he said.
The officer had noticed the man driving eratically and was following him.
Six officers joined firefighters in rescuing the man, who was under water for six to 15 minutes.
The man is now in serious condition, Pattison said.
The pond is located in north Edmonton about one kilometre west of 97th Street.