Intraperitoneal administration in a rat = is to inject the prepared dose into the peritoneal space,you pinch up the abdominal skin --> insert the needle through the skin --> reaching the space --> inject the dose " be sure that you don't pierce the abdominal organs , be careful and be sure you are in the peritoneal space then inject""""" ---> absorption of drugs via peritoneum to reach systemic circulation with high bioavailability.

Intravenous administration in a rat = is practically done via " rat tail vein". A proper technique of sterilization, dissection at the region of tail till properly insert your needle in the vein --> administration of a single dose/ injection. Be sure of proper and effective sterilization to avoid sepsis.Drug bioavailability is higher than the intraperitoneal administration reaching 100%, however the intraperitoneal is more convenient and simple than the intravenous injection.

Your choice depends on the type of your laboratory research, as the intravenous injection is prefered in acute studies and pharmacokinetics calculations.

Both types of administrations require good laboratory training.

Please refer to the following link:

http://www.usp.br/bioterio/Artigos/Procedimentos%20experimentais/Routeadministration-4.pdf

Best of luck

Intraperitoneal injection is done by passing a needle into peritoneum, whereas intravenous injection is done by passing a needle into a vein. The drugs injected via the two routes are fastly distributed. However, a drug injected via intravenous route is assumed to be 100% bioavailable unlike the other routes of administration.assoc

I confirm the Saganuwan' answer. In addition, the i.p. injection is a more simple and safe method of injection as well as its absorption rate is slightly slower but more persistent compared to the i.v. injection.