They slow down and spread out in the heat. Their fur becomes tumbleweeds throughout the house. They can be okay at higher temperatures, barring issues like age, ability, kinds of noses that prevent proper panting, etc. Before i turn the ac on, sometimes i'll see them spread out on the kitchen tile.

Supposed to be 40-45C (113F) in Toronto next week. We put ice blocks in the cage for our ratties.