1. Cholecystokinin (CCK) is released during stress both in limbic and hypothalamic areas suggesting that CCK could participate in modulating neuroendocrine as well as behavioural responses to stress. 2. In this study we have examined the effect of CCK receptor antagonists on the retention of the immobility response to a forced-swim stress in rats. In this test, rats are forced to swim during 15 min (conditioning period) and 24 h later, the duration of immobility is measured during a period of 5 min (re-test period). During the conditioning period rats display a period of vigorous activity, followed by progressive inactivity. During the re-test period rats remain 70-80% of the time in an immobile posture. 3. The CCKA receptor antagonist, devazepide (MK-329) but not the CCKB receptor antagonist, L-365,260, administered s.c. immediately before the conditioning period, decreased the duration of acquired immobility during the re-test period. The effect of devazepide was prevented by cholecystokinin octapeptide (CCK-8; 40 micrograms kg-1, s.c) as well as by the selective glucocorticosteroid GII receptor agonist, dexamethasone (30 micrograms kg-1, s.c.) 4. Neither corticosterone nor ACTH plasma levels measured both after the re-test period and after the conditioning period were modified by devazepide treatment. 5. The results suggest a role for CCK in the behavioural adaptation to stress and indicate a relationship between CCK systems and glucocorticoids in the neuronal mechanisms involved in the acquisition of adaptive behaviours to stress.