SUMMARY Evidence for the presence of cortisol and corticosterone in equine plasma and cortisol in equine urine is presented. The mean morning concentration of plasma cortisol was 7 μg/100 ml, and that of corticosterone 0·5 μg/100 ml. A diurnal rhythm of small amplitude was found. Administration of corticotrophin, induction of hypoglycaemia and surgery produced marked increases in plasma cortisol levels, but only small increases were seen after normal exercise. The changes in plasma cortisol were not invariably accompanied by significant alteration of the eosinophil count. It is concluded that the mechanisms of control of adrenocortical function in the horse are not dissimilar to those described for other mammalian species.