The effects of cold exposure on the release of thyrotropin-releasing hormone (TRH) and catecholamines as estimated by push-pull perfusion of the mediobasal hypothalamus were studied. Before cold exposure, the male rats had been kept at room temperature or at 30 °C for 3 weeks. Transfer to 4 °C increased plasma levels of thyroid-stimulating hormone (TSH), but this cold-induced TSH response was more pronounced in animals which had been acclimatized to 30 ° C. Exposure to 4 ° C also increased plasma thyroid hormone levels, but had no effect on plasma prolactin. The hypothalamic content of TRH and dopamine remained similar after transfer to 4 ° C, but after 6 h of cold, the content of noradrenaline and adrenaline had increased 1.6-fold and 3-fold, respectively. In vivo hypothalamic release of TRH, adrenaline and dopamine remained similar during a 2-hour period in control rats kept at room temperature or 30 ° C. The hypothalamic release of TRH, dopamine and adrenaline did not change in rats transferred from room temperature to 4 ° C. The amount of dopamine and adrenaline in push-pull perfusate also remained similar in rats acclimatized to 30 °C after transfer to low temperatures. However, in these rats kept at 30 °C for 3 weeks, exposure to 4 ° C increased TRH release in perfusate from the mediobasal hypothalamus in the first 15 min of cold exposure (2-fold increase). Thus, exposure to cold stimulates the hypothalamo-pituitary-thyroid axis and increases the hypothalamic release of TRH in rats which had been acclimatized to 30 <sup>¤</sup> C.