In vivo (Latin for (with)in the living). This term normally refers to research performed on whole organisms (e.g. a mouse). Clinical trials are a form of in vivo research, albeit on humans.

In vivo is a delineation which differentiates whole organism research from in vitro research, which is performed on organs, tissues, cells, cellular components, proteins, or biomolecules. The delineation is also useful to describe the mind-set of the researchers and the experimental models used.

In vitro research allows one to deduce mechanisms of action, rather than overall effect. Whole organisms are more complex than a subset of their components. In vitro research aims to describe and understand the effect of an experimental variable on a subset of an organism's components. In vitro research has the advantage over in vivo research that there are fewer variables which can confound an experiment, and that if an experimental effect is subtle the result will be more clearly visible.

In vivo research has the advantage, over in vitro research, that the experimental system is a more complex biological system. This means that in vivo research will likely give a better indication of what will happen in a population when a compound is administered or a procedure is performed. This is why all new drugs must first undergo animal testing, followed by clinical trials, before they are released to the general population.

The massive adoption of low-cost, in vitro, molecular biology techniques has caused a move away from in vivo research, which is considered too idiosyncratic and, above all, expensive compared to its molecular counterpart. Currently, in vitro models and experiments are a vital and highly productive research tool.

The guinea pig was previously such a commonly used in vivo experimental model that they became part of idiomatic English: 'being a guinea-pig for someone/something'. Their use in research has been substantially replaced by the smaller, cheaper and faster breeding rats and mice.

As the term is in Latin, it is written in italics.

See also: ex vivo, in utero, in situ, in vitro, in silico.