Three places or occasions that I can think of. 1) In summer, I no longer take dogs for full day hikes above treeline although many people do with no ill effects. Moosilauke would be OK provided it's not overrun, but, say, the Ammo loop would not. I think the heat stress from an extended period without shade, little water, and reflection of sun off rocks plus the wear and tear on the pads just isn't worth it, especially if the dog is older. (I once hiked a dog across Franconia Ridge under such conditions, and her tongue was literally blue by the time we got down to the hut. She wasn't one of those few naturally blue-tongued dogs either. We had to rehydrate her quickly and it wasn't easy to force a dog to drink in that condition.) 2) I also avoid the popular hikes--Zealand, Lonesome Lake, Bridal Veil--on the busiest days of the summer (but then I don't want to be there either). On the "tourist hikes," there are too many people and especially smaller children who get anxious at the sight of even well-behaved dogs to make it worth it. 3) And there are a very few places with ladders and pins and chimneys and such that I wouldn't take a dog that I couldn't lift. Some spots on Mansfield (chin, I think), come to mind. Heat stress is the big problem. On a day that I know may be in the 90s with humidity to match, if I hike at all, I would seek out a water trail (e.g. Cascade Brook Tr.) and leave very early in the morning so that the hottest part of the hike would be done well before noon. Each dog is different, of course. Some tolerate heat better than others. I'd just start out slowly and make sure you get a feel for your dog's level of tolerance. They can only pant, not sweat. Remember the tick problem.
