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The Channel Catfish Ictalurus punctatus is an important ecological, recreational and aquaculture species. In both natural and pond culture systems, Channel Catfish are periodically exposed to acute increase in temperature, especially in the summer, which may induce a stress response. With the effects of global climate change, the impact of rising temperatures on aquatic systems is expected to be exacerbated in the future. Hence, there is the need to understand the physiological mechanisms controlling the thermal stress response in fish. Acid-base, haematological, osmotic and metabolic regulations were investigated following acute temperature increase from 28 degrees C to either 32 degrees C or 36 degrees C at a rate of 2 degrees C/hr to assess the physiological response of Channel Catfish to temperature increase. Blood was sampled 1 hr after treatment temperatures were reached. When temperature increased from 28 to 32 degrees C, blood bicarbonate (HCO3-) decreased while cortisol, partial pressure of carbon dioxide (pCO(2)) and glucose increased. When temperature increased from 28 to 36 degrees C, blood pH, HCO3-, chloride, total proteins and mean corpuscular haemoglobin concentration decreased while cortisol, pCO(2), haematocrit, haemoglobin, glucose, lactate, mean corpuscular volume and mean corpuscular haemoglobin all increased. The changes observed are characteristic of acute stress responses in fish and suggest that acute temperature increases to both 32 and 36 degrees C are physiologically stressful to Channel Catfish.