Research conducted in a living organism — a necessary step between cell culture and human trials.Latin: "in life." Animal (rodent, primate) in vivo data accounts for absorption, metabolism, distribution, and whole-organism responses that in vitro data cannot capture. Rodent models have produced many false positives for human efficacy — particularly in CNS, oncology, and metabolic disease research. The translation failure rate from rodent to human is high enough that even compelling in vivo data requires cautious interpretation as a predictor of human outcomes. See: human studies gap.