In the rat and other small rodents, the very small muscle mass makes intramuscular administration both technically difficult and painful for the animal because of the distension of the muscle.
If intramuscular injections are necessary, they can be made into the front or back of the thigh in all small rodents. In the rat and guinea pig the muscle mass is usually sufficient for accurate administration of small volumes of material, ideally 0.05ml or less.
It is not usually necessary to try to sterilize the skin with antiseptics – their use is almost always ineffective and they prolong the duration of restraint needed and may cause additional disturbance to the animal.
As with all injection sites, using a new needle for each animal, and injecting fluid that is at body temperature will reduce any discomfort caused by the procedure.
Material that is irritant or with a high or low pH can cause pain both during and following injection.