Groups of adult male rats were decapitated without anaesthesia 30 seconds or 5, 10, 15 and 60 min after disturbance stress (investigators entering the animal room and moving the cages). The serum concentrations of LH, FSH, TSH, prolactin, triiodothyronine (T3) and thyroxine (T4) were measured by radioimmunoassay and corticosterone by a fluorometric method. With regard to the hormone levels measured in serum obtained within 30 seconds after induction of disturbance stress to resemble most closely the actual unstressed levels of endogenous hormones in circulation, serum corticosterone levels increased within 5 min. indicating that the procedure was stressful to the animals. In addition the serum prolactin and TSH levels were significantly elevated within 5 min, T3 within 60 min. Whereas corticosterone reached peak levels after 15 min. the serum levels of prolactin, TSH and T3 were still rising after 60 min. The FSH levels remained rather stable during the first 10 min. but started to rise during the following 5 min. At 60 min FSH levels were back to normal. Serum LH and T4 showed only minor fluctuations during the experimental period. These results indicate, that not only is the pituitary-adrenal axis stimulated by emotional stress, but also the pituitary-thyroid axis. It also seems, that emotional stress leads to a general activation of pituitary hormone release. Hence, proper care should be taken with regard to animal keeping, handling and the method of blood collection when dealing with rats as experimental animals.