Drugs to be administered to mice can be given orally (water, feed) or injected through a variety of routes. The average daily consumption of water and feed for an adult 25 g mouse is 4 ml and 3-5 g respectively.
3.1 Oral Gavage (0.1-0.3 ml for an adult 25 g mouse):
Gavaging is used to dose an animal with a specified volume of material directly into its stomach. Oral gavage is performed using a ball ended feeding needle. Estimate the distance that the needle needs to be inserted into the mouse (usually from the nose to the first rib) and mark it on the needle. Fill the syringe with the appropriate volume of material and attach the needle. Restrain the animal by the scruff. Place the tip or ball of the needle into the animal mouth. Slide the tip gently past the back of the tongue. The needle should slide easily down the esophagus, if properly placed (do not force and aspirate, if the animal struggles or appears to be in respiratory difficulty or any resistance exist, remove the needle and reinsert). Once the needle is properly placed, administer the material.
3.2 Injection:
Subcutaneous Injection (2-3 ml for an adult 25 g mouse): A new needle should be used for each animal, since this will reduce reduce the risk of infection at any injection-site. The injecting fluid should be at body temperature to further reduce the discomfortable. Restrain the mouse by the scruff method (Grasp the scruff The scruff can be grasped between the thumb and forefinger whilst). Prepare the injection site with 70% ethanol. Insert the needle, bevel up, at the base of the tent. The needle should be inserted parallel to the skin and should be directed toward the posterior of the animal. Aspirate to ensure proper placement and inject the material.
Intramuscular injections (0.03 ml per site): Restrain the mouse by the scruff method. Swab the area to be injected with 70% ethanol. Insert the needle into the caudal thigh, bevel up. Aspirate and inject. Since the foot has not been secured, make sure the mouse does not kick.
Intravenous Injections (0.5 ml for an adult 25g mouse): Warm the mouse under a heat lamp or other heating device, being sure not to overheat the animal (the temperature should not exceed 30~32°C ). Place the animal in a restraint device and stabilize the tail between the thumb and forefinger of the hand that will not be manipulating the syringe. Prep the tail with 70% ethanol. Attempt the injection starting at the middle or slightly distal part of the tail. With the tail under tension, insert the needle, bevel up, approximately parallel to the vein and insert the needle at least 3 mm into the vein. Inject the material in a slow, fluid motion. If any swelling at the injection site or resistance to injection occurs, remove the needle and reinsert it slightly above the initial injection site.
Intraperitoneal injection (2-3 ml for an adult 25 g mouse): To perform an IP injection, the mouse must be well restrained so that it cannot move during the procedure. This avoids traumatizing the organs once the needle has entered the abdomen. Restrain the mouse and tilt so that the head is facing downward and its abdoment is exposed. Insert the needle into the abdomen at about a 30-degree angle after disinfecting injection site. The shaft of the needle should enter to a depth of about half a centimeter. Aspirate to be sure that the needle has not penetrated a blood vessel, the intestines, or the urinary bladder. Greenish brown aspirate indicates needle penetration into intestines. Yellow aspirate indicates needle penetration into the bladder. If any fluid is aspirated, your solution is contaminated and must be discarded and the procedure repeated with a new syringe and needle.
Intradermal injection: Swab the injection site with 70% ethanol. Insert the needle into the skin, bevel up, holding the needle nearly parallel to the plane of the skin.  Do not aspirate. Inject the material. The volume of the injection should be limited to 50~100 ul per site to avoid tissue trauma. A properly performed intradermal injection will result in a small, round skin welt, which will maintain more than 15 minutes.
3.3 Nasal administration: Through nasal administration, a drug molecule can be transferred quickly across the thin mucosa directly to the systemic blood circulation without first-pass hepatic and intestinal metabolism. Nasal administration can therefore be used as an alternative to oral administration of for example tablets and capsules if a fast effect is desired or if the drug is extensively degraded in the gut or liver. Drug which shows poor absorbtivity can be given by this route. However, nasal administration is primarily suitable for potent drugs since only a limited volume can be sprayed into the nasal cavity. Drugs for continuous and frequent administration may be less suitable because of the risk of harmful long term effects on the nasal epithelium.
3.4 Pulmonary drug delivery: In preclinical animal studies, intratracheal instillation has frequently been used to assess the pulmonary absorption and systemic bioavailability, especially with regard to the precise dosing and effectiveness associated with this method. However, this method is more costly and difficult to measure the exact dose in lungs.
3.5 Transdermal: Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution. You could put the mice tail immersed in liquid, or apply drug liquid to the mice skin with a glass rod.