Abstract
Common, often overlooked, variables in biomedical research with animals are reviewed. The barren primary enclosure is an abnormal living environment for laboratory animals. Species-appropriate enrichment attenuates some of the distress resulting from chronic understimulation. Social deprivation distress of individually-caged social animals is best mitigated by the provision of compatible companionship. Biotelemetry and positive reinforcement training avoid or minimize stress reactions that typically occur when animals are forcibly restrained during procedures. The variables, 'light' and 'position of living quarters' are inherent in the multi-tier caging system. To date there is no satisfactory alternative other than the single-tier cage arrangement that eliminates both variables. Removing test animals from their familiar home environment and from their cage mates for procedures introduces stress as an avoidable influential variable. Music may become an important variable if not all subjects are exposed to it. Disturbance time cannot be controlled as an extraneous variable but it should at least be mentioned to explain possible incongruities of data. A positive relationship between animal care personnel and research subjects is a key requisite to minimize stress as a data-confounding variable.