Fifteen horses were rescued.
Firefighters arrived at the scene of the blaze at about 3 a.m. and found one barn fully engulfed in flames, and another also on fire.
Some of the surviving horses were taken to the stables at Exhibition Place.
The Sunnybrook Stables offer private and group horse-riding lessons, as well as summer camps.
Fire services says they're investigating the cause of the fire.
Toronto police asking if anyone saw suspicious activity in the area to contact them.