Heat stress profoundly impairs tolerance to central hypovolemia in humans, via a number of mechanisms including heat-induced hypovolemia. However, heat stress also elevates plasma osmolality; the effects of which on tolerance to central hypovolemia remain unknown. This study examined the effect of plasma hyperosmolality on tolerance to central hypovolemia in heat stressed humans. Using a counterbalanced and cross-over design, 12 subjects (1 female) received intravenous infusion of either 0.9% (ISO) or 3.0% (HYPER) saline. Subjects were subsequently heated until core temperature increased ~1.4°C, following which all subjects underwent progressive lower-body negative pressure (LBNP) to pre-syncope. Plasma hyperosmolality improved LBNP tolerance (ISO: 288 ± 193 vs. HYPER: 382 ± 145 mmHg x min, P=0.04). However, no differences in mean arterial pressure (P=0.10), heart rate (P=0.09), or muscle sympathetic nerve activity (MSNA, P=0.60, n=6) were observed between conditions. When assessing individual data, LBNP tolerance improved ≥25% in 8 subjects, but remained unchanged in the remaining 4 subjects. In subjects who exhibited improved LBNP tolerance, plasma hyperosmolality resulted in elevated mean arterial pressure (ISO: 62 ± 10 vs. HYPER: 72 ± 9 mmHg, P<0.01) and a greater increase in heart rate (ISO: +12 ± 24 vs. HYPER: +23 ± 17 beats/min, P=0.05) prior to pre-syncope. No differences in these variables were observed between conditions in subjects that did not improve LBNP tolerance (all P≥0.55). These results suggest that plasma hyperosmolality improves tolerance to central hypovolemia during heat stress in most, but not all individuals.