We determined the differential effects of environmental hyperthermia and reduced feed intake on circulating prolactin in growing pigs. Furthermore, we evaluated the correlation between prolactin concentration and other metabolic, productive and intestinal parameters during heat stress. Our results indicate that 1) hyperthermia elevates prolactin, and 2) prolactin participates in both the acute heat stress response and the acclimation to environmental hyperthermia. A better understanding of the physiological alterations produced by heat stress is essential in order to develop alleviating strategies to prevent the decrease in production and lost revenue during the warm summer months.