In this study, we aimed to investigate the acute stress responses of Black Sea trout to acute thermal fluctuations. Black Sea trout (184.22 g mean weight) were transferred from a private trout farm to a research center and restocked in a fiberglass square tank. After acclimation to culture conditions (at a water temperature of 15 degrees C) for 2 weeks, the fish in the experiment were exposed to 25 degrees C water temperature for 30 min, and the stress responses of the fish were examined at 1, 3, 6, 12, 24, 36, 48, and 72 h. After acute thermal shock, cortisol and glucose levels increased during the first hour in the experimental group. However, cortisol and glucose levels returned to their normal values after 6 and 36 h, respectively. Lysozyme activity decreased at 3 h and rose to its normal level at 12 h after the experiment. Total protein and serum ion concentrations were also affected and fluctuated due to acute thermal stress. Short-term acute thermal exposure also caused a serum ionic imbalance in the trout, but homeostasis was restored at 72 h.