Adult male wild rats, Rattus norvegicua, were exposed, for 10 to 60 mia, to attack by a fierce male of the same species. There was little or no wounding, yet 15 out of 21 died within 6 days of exposure. Ten controls put in an empty cage for similar periods all survived. In 7 rats that survived for long enough, thyroid secretion, recorded by means of I131 release curves, was reduced for about 34 hr. Six other rats died during a period of thyroid inhibition. The period of inhibition began 4 to 59 hr. after exposure to attack. Ten controls showed no inhibition. In similar rats, similarly attacked, reduced adrenal ascorbic acid indicated raised adrenal cortical activity. The thyroid inhibition was evidently a secondary result of central nervous arousal, perhaps mediated by a decline in the output of thyroid stimulating hormone.