Unlike research using mammalian animal models such as rats or mice, experimental fish often come from wild or commercial sources, leading to a lack of well defined experimental animal models. Isogenic carp offer us a well defined fish model for physiological research. The aim of this thesis was to, therefore, investigate the physiological stress response of isogenic strains of carp. Increased levels of stress in intensively reared animals results in large economical losses (due to disease mortalities, poor growth) and an increased understanding of the stress response is therefore relevant to the fish farming industry. Using a standard stressor, we aimed to examine the influence of environmental disturbances (e.g. high densities, restricted feeding levels) on the physiological response of the carp to this stressor. Fish showed a mild response to the period of crowding, although they appeared to recover physiologically, fish reared at a high density were more sensitive to an additional disturbance as seen by the higher levels of stress-hormones in the circulation. In addition, alternate periods of optimal or maintenance feeding levels were also found to affect the response to stress. An alternative method for measuring chronic stress in fish was also established through hormonal measurements in the water. A stress disorder was noted in one strain, these fish appear to suffer from a disorder similar to the 'chronic adrenal hyperplasia' which occurs in mammals. As this has never been described in a lower vertebrate, these fish may prove to be an important model for future studies on fish endocrinology.