Thermal stressors such as low and high ambient temperature elicit an abundance of neuroendocrine responses including activation of the hypothalamo-pituitary-adrenal (HPA) axis, and arginine-vasopressin (AVP) release. The exposure to heat is particularly interesting model for AVP action studying since this kind of stressor represents not only an unpleasant experience, but a threat to the osmotic homeostasis as well. As AVP has long been recognized as a hormone involved in the modulation of HPA axis activity, the aim of this study was to elucidate the AVP role in acutely heat exposed rats using Nelivaptan, a selective V1b receptor antagonist. Rats were exposed to high ambient temperature (38°C) for 60 minutes. The circulating hormones were determined by ELISA test or chemiluminescence method and Western blot was employed for determination of intrapituitary adrenocorticotropic hormone (ACTH) and V1bR level. The obtained results show that V1bR blockade negatively affected blood ACTH increase, caused by heat exposure. This treatment alone, or in combination with Nelivaptan decreased intrapituitary V1bR amount, while circulating AVP concentration was increased under the same conditions. Furthermore, strong correlation was observed between blood ACTH and corticosterone concentration. In conclusion, our results directly confirmed the positive role of AVP in the regulation of ACTH secretion from the pituitary in animals exposed to heat. Moreover, the results suggest that AVP from general circulation influence the pituitary V1bR.