My two really slow down quick when it's hot outside. Fortunately our AC has been good to us so far, that and we have a basement that stays incredibly cool. While the dogs don't spend a lot of time down there, it does mean that the floors on the first level are nice and cold. The fur-darlings will stretch out on the kitchen floor to cool off. We keep them well watered (if we don't, the toilets will be pillaged) so they don't get dehydrated. I am confident that a dog that has shade and enough to drink can't go into hyperthermia, but then again I'm not a vet...