The environment in which laboratory rodents are kept has an impact on experimental data and breeding performance. The quality of the environment is defined by both the quality of the primary enclosure and the secondary enclosure and their fluctuations. There are regulations and international recommendations for optimal values and ranges of most relevant environmental parameters, but it is actually the whole environment and animal care and use procedures what might (and certainly will) impact experimental results. Standardization of animal models is an issue of paramount relevance in biomedical research and this standardization must include procedures, genetics, health status, but also the environment animals are kept in. The presentation deals with environmental standardization and the impact that lack of control of these variables can have on experimental results.