This contribution will be concerned with the interaction between environment, adaptability optimization and behaviour. Animal laboratory experiments demand repeated measurements under identical environmental conditions. This is a prerequisite for the conventional statistical methodology used in order to clarify causal relationships involved in various biological functions. The understanding of biological functions is a necessary fundament for knowledge to prevent illness and to achieve a palliative or specific therapy. It is reasonable to assume that the routines in the quarters are very artificial, considering an animal's normal living conditions. The experimental situation as well as animal maintenance involves a process of adaptation. Adaptability depends on type of animal, degree of domestification etc. However, even with respect to choice of suitable species, strain and genetic manipulation, the process of adaptation becomes an important variable for ethical and practical points of view. The more emphasis on constancy, the more do we run the risk of increasing the span between normal and laboratory conditions and subsequently increase the factor and problem of adaptation. This vicious circle should be broken rather by finding optimal conditions than by a middle course determined by experimental requirements, economical frames and general notions about what may be good for the animal. Optimization must involve an understanding of how the experiment and the way of maintenance of the animal in the animal quarters influence adaptability. This understanding requires a systematic exploring of what physio-chemical and psychological factors are of importance. We will probably never be able to control the variability in the degree of adaptation.(ABSTRACT TRUNCATED AT 250 WORDS)