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Outside, create an enclosure or fenced in area for them to play in. You can use a wire fence or cage, as long as it doesn't leave gaps that your guinea pigs can escape through. Guinea pigs can't stand too much heat, so make sure you keep them cool when they go outside. For one, you should always provide them with shade, so they can cool down. In addition, only let them out during cooler times of the day in the summer. Guinea pigs prefer temperatures between 65 and 75 degrees Fahrenheit. While they can stand a bit more variation, you don't want to go too much colder or warmer. Damp grass can give your pig a chill, and that's not good for them. Also, make sure you aren't setting them down on grass that has been sprayed with insecticides or other chemicals, as that can be damaging for your guinea pig. Once again, you want to be sure to always provide your guinea pig with food and water. Water is especially important when your guinea pig is outside because it can easily get dehydrated when it is warm out. Guinea pigs love to chew on any plant in their vicinity, which is fine when they're munching on grass. However, some plants are poisonous to guinea pigs, so you want to make sure they don't have access to those.  Use a database like Cornell's http://poisonousplants.ansci.cornell.edu/index.html to look up poisonous plants in your area. Many common plants like daisies and buttercups are poisonous to guinea pigs. If you keep your guinea pigs just on grass without weeds, you should be fine. You do not want to leave your guinea pig unattended outside. For one, if your enclosure doesn't have a roof, your guinea pig could easily be carried off by a predator. It could even happen if your enclosure does have a roof.  In addition, you want to make sure your guinea pigs don't wriggle out and escape. Also, because guinea pigs are susceptible to heat stroke, you want to keep an eye on them while outside. Signs of heatstroke include lethargy, a limp body, trouble breathing, a rapid pulse, and a wet chin. Your guinea pig may also feel hot to the touch. Check the ears to see if they feel extra warm. If you notice signs of heatstroke, bring the guinea pig indoors. Use tepid water (not cold because it can cause shock) to dab around the ears and feet. You can even dip its feet in water. Once the guinea pig seems cooler, take it to the vet.

Summary:
Place them in an enclosure. Keep your guinea pigs cool. Don't let them go out on damp grass. Provide food and water. Check for poisonous plants. Stay outside.