Gerbils reared in tunnel systems responded to a visual stimulus by fleeing, foot-thumping and remaining concealed, whereas many gerbils reared in laboratory cages responded in the same situation by approaching the stimulus. The critical factor in tunnel-rearing was the opportunity to flee to shelter during maturation. Neither isolation from illumination nor isolation from stimuli associated with human handlers produced the observed effect. Gerbils reared in laboratory cages exhibit the pattern of flight and concealment in response to stimulation following 24-hr experience in a tunnel system. The data are discussed in terms of their implications for models of the ontogeny of the behaviour characteristic of domesticated, as compared with wild, strains.