The most common natural rat repellent is peppermint oil. To be effective, the oil must be highly concentrated. It must also be reapplied whenever the smell is no longer noticeable upon first entering the treated area.
It is also effective to mix garlic, horseradish and cayenne pepper in vegetable oil and spray it around areas where rat droppings have been observed. The best way to keep rats out of a building or home is by keeping everything clean. Food and crumbs left on the table or floor are likely to attract rodents. Daily sweeping and the prompt removal of garbage goes a long way toward preventing a rat infestation.