Behavioral and adrenocortical responses to repeated mechanical restraint were compared in 28-day-old to 31-day-old male Japanese quail from two genetic lines divergently selected for reduced (low stress, LS) or exaggerated (high stress, HS) plasma corticosterone (C) responses to brief immobilization. Restraint in a metal crush cage for 5 min elicited immobility and silence in all the birds. Circulating C levels were considerably higher in quail of both lines following restraint than in the undisturbed controls of either line. As expected, both the behavioral and physiological effects were more pronounced in HS than in LS birds. Struggling increased with repeated restraint in HS and LS quail, thus suggesting behavioral habituation to the stressor in both lines. On the other hand, a line effect on the pattern of adrenocortical responses was revealed upon subtracting the change in plasma C concentrations from Day 1 to Day 4 in the undisturbed controls from the corresponding change in restrained birds. Thus, unlike LS quail, in which there were no detectable effects of repeated restraint, the adrenocortical responses of HS birds showed evidence of experience-dependent sensitization. Our results demonstrate the importance of the background genome in determining the patterns of the behavioral and adrenocortical responses elicited by repeated exposure to stressful stimulation. The present results and those of previous studies could be explained in one or both of two ways: that underlying fearfulness is lower in LS than HS quail or that they adopt active or passive coping strategies, respectively. Our findings may also have important implications for poultry welfare and productivity. @ 2000 Elsevier Science Inc.