In vivo means "within the living": experiments performed in an intact organism rather than in culture. In peptide research the term almost always denotes rodent work — mouse and rat — with zebrafish, rabbit, dog and non-human primate models appearing at later stages.
What in vivo adds, and what still limits it
An in vivo study puts every process a cell assay removes back into play at once: absorption from the administration site, plasma protein binding, tissue distribution, hepatic and renal clearance, peptidase exposure, immune recognition and homeostatic feedback. A peptide that binds its receptor at nanomolar affinity in vitro may never reach that receptor in an animal, and this is where a large share of peptide programmes stop.
Interpretation still requires care. Model validity is the usual weak point — an induced rodent model resembles the human condition it is named after only partially. Species differences in receptor sequence, metabolic rate and peptidase profile all shift results, and rodent-to-human scaling is not a simple ratio. Study quality varies: randomisation, blinding and sample-size justification are inconsistently reported in the peptide literature.
Note the framing rule that applies to this site: in vivo findings are described by species and model, and they never support statements about people. Material here is research use only.
Related terms and material
preclinical · phase 1/2/3 trial · species reactivity. Reference material: all research products. Further reading: what research use only means.