Educational reference only, not veterinary advice. Rats cannot sweat. They lose heat through the tail and ears and by spreading saliva on their fur, and they overheat far more easily than people expect. Above roughly 26-27C most rats start to struggle; heat stroke can be fatal within hours. Prevention: keep the cage out of direct sun and away from conservatories and south-facing windows. Move rats to the coolest room of the house in a heatwave - often downstairs or a hallway. Use a fan aimed near the cage but never directly at the rats, and close curtains during the day. Cooling ideas: frozen water bottles or wrapped ice packs placed against the outside of the cage or under a covered corner; ceramic or granite tiles chilled in the fridge; a shallow dish of cool water with peas to bob for; frozen fruit or watermelon; a damp towel over part of the cage top so air passing through is cooler; extra water sources. Remove deep nesting material and fleece hammocks in extreme heat and offer cool, open hides instead. Warning signs of heat stress: lying flat and stretched out, panting or open-mouthed breathing, drooling and wet fur around the neck, weakness, unresponsiveness. Move them somewhere cool, offer water, apply cool (not cold) damp cloths and contact a vet immediately. Never immerse a rat in cold water - the shock can kill. Older, overweight and unwell rats are the first to suffer. Watch them closely.