Traditionally it has been thought necessary to lose 2% of body mass due to dehydration, before functioning is disrupted, although recently adverse effects have been reported with a loss of 0.5-0.7%. It is, however, unclear whether the response to small decreases in mass reflects dehydration, mechanisms that help to adapt to a loss of bodily fluid, or a placebo effect. Individuals were therefore subject to a temperature of 30°C for three hours, and mood and cognition monitored. To explore the possibility of a placebo response, the consumption of plain or coloured water was compared. To consider changes in hydration status, drinks known to differ in their ability to rehydrate were contrasted. Not drinking was disruptive, although a combination of plain water and electrolyte’s most effectively prevented a decline in functioning, indicating a role for rehydration after a loss of 0.52% body mass. There was, however, also evidence of a placebo response: a combination of plain water and electrolytes was better able to prevent a decline in functioning than coloured plain water and electrolytes. As increased anxiety was a robust response, it was discussed whether the renin–angiotensin–aldosterone system might be part of the mechanism.