Environment Canada has issued a heat warning for Bracebridge, Gravenhurst, Huntsville, Baysville, Port Carling, Port Severn, the Town of Parry Sound, Rosseau and Killbear Park.
According to the alert, significant heat and humidity will arrive on Friday.
Daytime temperatures in the low to mid-30s with afternoon humidex values of 40 or higher are expected for Friday and Saturday.
Nighttime minimum temperatures in the low to mid-20s are predicted for Friday and Saturday.
A cooler and less humid air mass is forecast to arrive in the wake of a weak cold front on Sunday, the warning says.
Environment Canada says although extreme heat affects everyone, the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.