Police say a four-year-old girl called 9-1-1 on Friday after her babysitter became unresponsive while in the deep end of a backyard swimming pool in Newmarket.
York Regional Police say they were called to the residence in the area of Davis Drive and Alexander Road around 5 p.m.
They say paramedics performed CPR and she was taken to hospital in life-threatening condition.
Police are investigating the incident but say they do not consider it to be suspicious.