Measurements were sought for assessment of stress during investigations of welfare in intensively housed poultry. Immobilization was used as a stressor in experiments involving 64 cockerels of various ages. Two parameters were found to be related to acute stress: plasma triiodothyronine (T3) concentration, which fell markedly, and plasma corticosterone concentration, which rose considerably, in all birds during severe stress. Concurrently, plasma thyroxine (T4) concentration also fell in most, but not all, birds. Thyrotrophin releasing hormone (TRH) injected intravenously after immobilization caused increases in T3 and T4 levels, suggesting that the fall in T3 and T4 during acute stress was not caused by exhaustion of hormone production by the thyroid gland but rather by inhibition of release of TRH at the hypothalamic level.