The stress response of common carp Cyprinus carpio was studied by evaluating plasma cortisol, glucose and lactate after single or multiple rapid temperature drops (ΔT: 7, 9 or 11°C). All three amplitudes used induced a significant rise in plasma cortisol levels. Peaks occurred within 20 min after onset of the cold shock. No stress-related secondary metabolic changes were observed in any of the experiments described: plasma glucose levels remained unaffected and plasma lactate levels dropped. Carp of 60 days old showed a significant stress response, although plasma cortisol levels were lower than those observed in carp of 120 days. Furthermore, fish that had experienced multiple cold shocks showed an overall lower cortisol response than fish experiencing a single cold shock, indicating that habituation to this stressor occurred. (C) 2000 The Fisheries Society of the British Isles.