Intraperitoneal (IP) injection is the administration of a liquid substance into the peritoneal cavity — the space within the abdominal cavity that surrounds the digestive organs — of a laboratory animal. In mice and rats, IP injection is performed by inserting a needle through the lower abdominal wall, directing the tip away from the viscera and into the free peritoneal space. Compounds administered IP are absorbed into the systemic circulation via the peritoneal capillaries and lymphatic vessels, typically producing onset of action within minutes to a few tens of minutes depending on the compound.
IP injection is one of the most commonly used dosing routes in rodent research because it is technically straightforward, allows relatively large volumes to be administered, and produces systemic exposure more rapidly than subcutaneous injection. It is the standard route for administration of anaesthetic combinations such as ketamine-xylazine in mice and rats, and is widely used for pharmacological, immunological, and oncological study compounds.
The injection site is the caudal right quadrant of the abdomen — the lower right region of the animal. This location is preferred because it avoids the liver (upper right quadrant, though it spans cranially), the caecum (lower left, especially in mice where the caecum is large and sits predominantly on the animal's left side), and the urinary bladder (midline). The animal is restrained in dorsal recumbency (on its back) with the head tilted slightly downward so that the abdominal organs shift cranially, increasing the space available in the injection zone. The needle is inserted bevel up and the plunger is aspirated before injection. For both mice and rats, an angle of approximately 30–45° to the abdominal wall is standard; because the rodent abdominal wall is thin, the needle should not be advanced deeply once the peritoneum is entered, to avoid driving the tip into the viscera. A common cause of subcutaneous misdelivery is insufficient needle penetration — the tip remains in the subcutaneous layer rather than crossing the abdominal wall. Aspiration of blood indicates intravascular placement; aspiration of pale yellow fluid indicates bladder puncture (urine); aspiration of green or greenish-brown fluid indicates intestinal or caecal puncture. In any of these cases, withdraw the needle and use a fresh needle and syringe.
Needle gauge for IP injection: 25- to 27-gauge is standard for mice; 23- to 25-gauge for rats. Needle length should be sufficient to penetrate the abdominal wall without excessive depth — 16 mm (5/8 inch) needles are common for mice, while longer needles are used for rats. For mice, the standard recommended volume is up to 10 mL/kg body weight for a single IP dose; a 25 g mouse can typically receive up to 250–500 µL. Some institutional guidelines cite 20 mL/kg as an absolute upper ceiling for isotonic, non-irritant solutions, but 10 mL/kg is the routine working limit. For rats, up to 10 mL/kg per dose is generally accepted.
The main complication risks in IP injection are inadvertent organ puncture — particularly of the intestines, caecum, or bladder — and subcutaneous delivery if the needle does not penetrate the peritoneum fully. Organ puncture can be minimised by using the correct injection site and angle, ensuring the animal's head is tilted downward to shift viscera away from the injection zone, and using appropriate needle length. If the animal shows signs of peritonitis or abnormal distension in the hours after injection, veterinary assessment is required.
Key Points
- Inject into the caudal right quadrant of the abdomen to avoid the liver, caecum (lower left in mice), and bladder
- Aspirate before injecting — blood = intravascular; yellow = bladder (urine); green/brown = intestinal; all require repositioning
- 25–27 gauge for mice (26–27G preferred); 23–25 gauge for rats; angle ~30–45° for both mice and rats; bevel up
- Volume guideline: up to 10 mL/kg (routine limit) for both mice and rats; 20 mL/kg is an absolute ceiling for mice only
- Tilt animal head-down to shift viscera and increase safe injection space
Relevant Standards
- OECD Test Guidelines (specify IP as an acceptable dosing route for many acute and subacute toxicity studies)
- ARRIVE Guidelines (reporting standards for animal research — require documentation of route, volume, and site)
- Institutional animal ethics committee (AEC in Australia; IACUC in North America) protocols govern approved volumes and technique
Related Terms
Frequently Asked Questions
Why is the caudal right quadrant used for IP injection in mice?
In mice, the liver occupies the upper abdomen (predominantly right side) and the caecum (a large blind-ended intestinal pouch) sits in the lower left quadrant in approximately 80–85% of mice. The urinary bladder is in the midline. The caudal right quadrant is therefore the region with the fewest critical structures in the immediate needle path, making inadvertent organ puncture least likely. Tilting the animal's head down shifts the organs cranially, further clearing the injection zone.
How do I know if I accidentally injected into the intestine?
Before injecting, aspirate gently on the syringe plunger. If green or greenish-brown fluid appears, the needle tip is inside the intestine or caecum; if pale yellow fluid appears, the bladder has been punctured (urine). In either case, withdraw the needle immediately without injecting. Discard the syringe and needle, prepare a fresh syringe, and attempt the injection again at the correct site. Intestinal puncture without injection of the compound is generally not a serious complication if the needle is fine-gauge, but delivering compound into the gut lumen changes its absorption and could produce toxic effects.
What volume can I inject IP in a mouse?
The standard recommended limit is up to 10 mL/kg body weight as a single IP dose, which for a 25 g mouse is approximately 250 µL. Some institutional guidelines cite 20 mL/kg as an absolute upper ceiling for isotonic, non-irritant solutions, but this is not a routine working volume — 10 mL/kg is the practical standard. Always use the minimum volume needed to deliver the dose, and confirm the approved volume with your institutional animal ethics committee.