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Increased levels of tissue monoaminergic neurotransmitters, as well as circulating catecholamines, appear to play a role in the regulation of the physiological responses of teleost fish. Harpagifer antarcticus is a stenothermic, Antarctic notothenioid fish. The aim of this study was to determine the effect of increased seawater temperature and decreased salinity on the levels of 5-HT, 5-HIAA, DA, and Noradrenaline in the brain, stomach, and gut of H. antarcticus. Wild-gathered fish were acclimatized to habitat conditions (2 degrees C, 33 PSU) prior to placement in aquaria with 4 temperatures (2, 5, 8 and 11 degrees C) and 3 salinities (23, 28 and 33 PSU) for 10 days. Fish exposed to 11 degrees C had higher levels of the brain neurotransmitters than those at 2 degrees C. Concomitant exposure to low salinity exacerbated the effect of exposure to 11 degrees C. At lower temperatures, concomitant alterations in salinity induced differential effects on brain neurotransmitters. When fish were exposed to 28 PSU, 5-HIAA, DA, and Noradrenaline levels at 5 and 8 degrees C presented no significant differences with those at 2 degrees C. In contrast, only 5HT and 5-HIAA levels in fish at 33 PSU were elevated at 5 and 8 degrees C respectively. Fish at 28 and 33 PSU had lower Gut 5HT levels at the 3 elevated temperatures, meanwhile fish at 23 PSU showed a biphasic effect when exposed to elevated temperatures. 5-HIAA levels decreased at 5 and 8 degrees C at 33 PSU. Stomach 5HT levels also showed a differential response at the 3 salinity levels when exposed to increased temperatures. At 11 degrees C, 5HT levels were markedly higher than those at 2 degrees C for fish at 33 PSU, moderately elevated for fish at 28 PSU, and lower for fish at 23 PSU, meanwhile 5-HIAA levels only increased with temperature at 33 PSU. These findings indicate that rapid exposure to alterations in temperate with or without concomitant changes in salinity is associated with differential responses in tissue monoaminergic neurotransmitter levels. The relatively high changes in neurotransmitter levels in fish exposed to moderate salinity and high temperature changes may indicate the physiological plasticity of H. antarcticus to possible changes in ocean temperature and salinity.