Large and rapid variations were found in the plasma cortisol levels of housed and cannulated sheep. Adrenaline injected i.v. caused increased plasma levels of cortisol that were proportionate to the dose. This response of cortisol to adrenaline was larger when sheep were newly housed, than when the sheep had been housed and sampled for 2 weeks. Response to adrenocorticotrophin also diminished over 2 weeks. Dexamethasone abolished the response to adrenaline. Tyrosine and DOPA had little effect on cortisol levels, dopamine and noradrenaline had some effect, but none had as great an effect as adrenaline.