An animal's response to a stressor is to increase metabolic rate, and thus energy consumption through the activation of the hypothalamic-pituitary-adrenal axis. Changes to energy use by an animal are likely to influence the efficiency with which it is utilised. In this study, we tested the hypothesis that less efficient sheep are more responsive to exogenous administration of adrenocorticotropin hormone. This was done by firstly determining the appropriate dose (0.4, 1.6 or 6.4microg/kg LW) and peak serum cortisol response time (45min) to exogenous administration of adrenocorticotropin hormone in a pilot study (n=3 sheep). Following this, adrenocorticotropin hormone (2.0microg/kg LW) stimulated cortisol levels were measured in a larger group of sheep (n=50) of known feed efficiency (feed conversion ratio and residual feed intake values). Less efficient sheep (more positive residual feed intake values) were found to have a greater (P<0.001) increase in cortisol concentration in comparison to more efficient animals. Those sheep which had higher levels of cortisol also had a greater proportion (P<0.001) of fat tissue. These data clearly demonstrated that efficiency of energy use, when measured as residual feed intake, is significantly related to an animal's stress response. These findings have important implications for understanding the physiological mechanisms underpinning efficiency of energy use, and may be useful in successfully identifying animals which are superior in terms of feed efficiency.