During exercise-heat stress, ad libitum drinking frequently fails to match sweat output, resulting in deleterious changes in hormonal, circulatory, thermoregulatory, and psychological status. This condition, known as voluntary dehydration, is largely based on perceived thirst. To examine the role of pre-exercise dehydration on thirst and drinking during exercise-heat stress, ten healthy males (21±1 yr, 57±1 ml^.kg^-1.min^-1 VO₂max) performed four randomized walking trials (90 min, 5.6 km^.hr^-1, 5% grade) in the heat (33°C, 56% RH). Trials differed in pre-exercise hydration status (euhydrated [EU] or hypohydrated to -3.8±0.2% baseline body weight [HY]) and water intake during exercise (no water [NW] or water ad libitum [W]). Blood samples taken pre- (PRE) and immediately post- (IP) exercise were analyzed for hematocrit, hemoglobin, serum aldosterone, plasma osmolality (P_osm), plasma vasopressin (P_AVP), and plasma renin activity. Thirst was evaluated at similar times using a subjective 9-point scale. Subjects were thirstier before (6.65±0.65) and drank more during HY+W (1.65±0.18 L) than EU+W (1.59±0.41 and 0.31±0.11 L, respectively). Post-exercise measures of P_osm and P_AVP were significantly greater during HY+NW, and plasma volume lower (HY+NW = -5.5±1.4%, HY+W = +1.0±2.5% [p=0.059], EU+NW = -0.7±0.6% [p<0.05], EU+W = +0.5±1.6% [p<0.05]) than all other trials. Except for thirst and drinking, however, no HY+W values differed from EU+NW or EU+W values. In conclusion, dehydration preceding low-intensity exercise in the heat magnifies thirst-driven drinking during exercise-heat stress. Such changes result in similar fluid regulatory hormonal responses and comparable modifications in plasma volume regardless of pre-exercise hydration state.