Abstract 1. 1. Resting cortisol values in immobilized mature Rusa stags ( Cervus rusa timorensis) and the response to synthetic ACTH were investigated. 2. 2. The mean level of cortisol in mature Rusa stags was found to be 3.80 ng/ml ( SD = 0.87, N = 18). 3. 3. Over the range 0.37–6.0 i.u. the adrenal response to ACTH challenge was linearly related to the log dose ACTH administered ( r = 0.998). 4. 4. More than 6 i.u. of ACTH caused maximal stimulation of the adrenal gland. 5. 5. Rusa deer appear to be much more sensitive to ACTH administration than other species.