In vivo (Latin: within the living) means that which takes place inside an organism. In science, in vivo refers to experimentation done in or on the living tissue of a whole, living organism as opposed to a partial or dead one or a controlled environment. Animal testing and clinical trials are forms of in vivo research.
in vivo research
This type of research approaches subject experimentation holistically. It is, often, better suited for observing the overall effects of an experiment on its living subject (see in vitro for its description and respective merits). In molecular biology "in vivo" may refer to experimentation done at the cellular level within the natural milieu of intact living cells. Once cells are disrupted and individual parts are tested or analyzed, this is known as "in vitro".
According to Christopher Lipinksi and Andrew Hopkins, fellows with Pfizer Global Research and Development, in vivo research has an advantage in that: Whether the aim is to discover drugs or to gain knowledge of biological systems, the nature and properties of a chemical tool cannot be considered independently of the system it is to be tested in. Compounds that bind to isolated recombinant proteins are one thing; chemical tools that can perturb cell function another; and pharmacological agents that can be tolerated by a live organism and perturb its systems are yet another. If it were simple to ascertain the properties required to develop a lead discovered in vitro to one that is active in vivo, drug discovery would be as reliable as drug manufacturing.
In the past, the guinea pig was such a commonly used in vivo experimental subject that they became part of idiomatic English: "to be a guinea pig for someone/something". However, they have largely been replaced by smaller, cheaper, and faster breeding rats and mice.