One technique used to assess chronic stress in animals is to determine how responsive they are to a corticotropin-releasing hormone (CRH) or an adrenocorticotropic hormone (ACTH) challenge. This is a physiological means of mimicking a severe stress. Animals that have been chronically stressed should have the capacity to respond to such challenges with a larger release of the stress hormone cortisol. This study examined the dose of CRH needed for such a test and the time frame over which a response should be measured. The study was done with 12-wk old calves. Calves were injected with varying doses of CRH and their responses in terms of ACTH and cortisol were assessed over the next 5 hours. The cortisol concentrations between 20 and 90 min post-injection was predictive of the total release of cortisol following treatment. It is suggested that a challenge test should include several samples during that period of time.