Fresh clean water must always be available so your rats don't dehydrate, maybe consider buying an extra bottle for your cage if you find that yours is nearly always empty at the end of the day. It really is surprising how much rats will drink , its far more then any other rodent I've kept.

Make sure your cage is out of direct sunlight, especially if you keep your rats in a tank. You know the phrase "dogs die in hot cars" well I can quite believe the same is true of rats in hot tanks or hot rooms.

If your cage is shut  away in a room make sure the windows and doors are open to allow good ventilation which will make the temperature drop considerably.

Avoid handling and exercise at the hottest parts of the day, try having your ratties out in the early morning or in the evening. There must be nothing worse for a rat then to be handled by hot hands on a hot day !

Avoid travelling in really hot weather unless you have to. If you do have to travel with your rats then try and use a car with air conditioning, if not make sure you have the windows down to ventilate the car (it is also important to do this before you start your journey, because a car sat in the sun can be stifling inside), keep all the rats in the shade and provide them with a source of water -bottles are better, as they cant be tipped over.

Remove all uneaten fruit/veg,  to avoid flies being attracted to your cage !

Regularly clean your rats litter trays, bedding and toileting corners to avoid build up waste which will again attract flies but will also decompose quicker in the hot weather thus giving off more ammonia which can irritate your rats respiratory tract. Waste in the cage will also pong more in the hot weather !

Give your rats less bedding so they don't get too hot in their houses ! and if you use a wooly bedding (which I would advise against anyway) change it to a shredded towel type bedding which will be less hot.

If you have enclosed plastic houses consider drilling holes in them to allow for better ventilation, this will also prevent condensation build up which can lead to damp bedding.

Consider buying your rats a small rabbit run, to go outside in the evenings, to cool off ! But remember you'll have to supervise them !

With confident rats, avoid putting in too many  houses/ nest boxes  which they will cram in and get hot !  (Nervous rats will not appreciate having no where to hide though).

Provide open areas in your cage or large open boxes (for more security) where your rats can just flop and stretch out !

Provide a water tray for your rats to paddle, drink and dabble in ! I use a cat litter tray and put two mini paving slabs in it to avoid them tipping it over when they perch on the edge and for them to sit on.

Places a electric fan in the room where you keep your  cage to keep the air moving and the room cool !

Put frozen veggie or fruit in your water tray to encourage your rats to use it, this also gives them a cooling treat ! (frozen sweet corn works well).

Give your rats ice cubes to play with ! these can also be put in your water tray.

Get a jam jar and fill it with Ice cube then put this in the cage for them to lay near if they need to cool down.

Make your rats fruity lollies ! Get an old yogurt pot and put some fruit in it e.g. chopped up apple and then add water till its an inch from the top and freeze! Alternatively you can add a little zero sugar squash (concentrated fruit drink) for an extra treat, but don't put too much as it will be too sticky and unhealthy.

Put clean paving slabs in your cage for somewhere cool for your rats to lie on and also wear there claws down.