The animal rights organization used Mathieu to show what happens when you leave a dog in a car on a hot day.
In the video, Mathieu was recorded drenching in sweat and fanning himself as temperatures crawled up to 120 degrees.
"If you're going to make a dog part of your family, then make him part of your family," Mathieu said in the video. "I seriously couldn't imagine leaving my dog in a car like this."
According to PETA, on a 78-degree day, the temperature inside a parked car can soar to 100 degrees in minutes.