Purpose: This study examined the effects of rehydration (Rehy) with fluids of varying tonicities and routes of administration, after exercise-induced hypohydration on exercise performance, fluid regulatory hormone responses and cardiovascular and thermoregulatory strain during subsequent exercise in the heat. Methods: On four occasions, 8 males performed an exercise-dehydration protocol of ~185 min [33°C] to establish a 4% reduction in body weight. Following dehydration, 2% of the fluid lost was replaced during the first 45 min of a 100 min rest period, by one of three random Rehy treatments [0.9% saline IV; 0.45% saline IV; 0.45% saline oral] or no Rehy [NF] treatment. Subjects then stood for 20 min at 36°C and then walked at 50% VO_2max for 90 min. Results: Subsequent to dehydration, plasma Na⁺, osmolality, aldosterone and arginine vasopressin concentrations were elevated [p<0.05] in each trial, accompanied by a -4% hemoconcentration. Following Rehy, there were no differences [p>0.05] in fluid volume restored, post-Rehy body weight or urine volume. %PV was 5% above pre-Rehy values and plasma Na⁺, osmolality and fluid regulatory hormones were lower compared to NF. During exercise skin and core temperatures, heart rate and exercise time were not different [p>0.05] among the Rehy treatments. Plasma osmolality, Na⁺, %PV and fluid regulatory hormones responded similarly among all Rehy treatments. Discussion: Neither a fluid of greater tonicity or the route of administration resulted in a more rapid or greater fluid retention, nor did it enhance heat tolerance or diminish physiological strain during subsequent exercise in the heat.