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To determine whether carbohydrate ingestion would reduce cognitive dysfunction in humans following long duration passive heat stress (PHS) versus consuming electrolytes alone. Fifteen young (27 ± 4 y) healthy adults were exposed to 120 min of PHS through the use of a liquid perfused suit (50 °C) on two randomized visits. Subjects consumed fluids supplemented with electrolytes (E) or electrolytes + carbohydrates (E + C). Pre- and post-heat stress, body mass (BM) and plasma osmolality (pOsm) were measured. Heart rate (HR), blood pressure (BP), Physiological Strain Index (PSI), core temperature (T c), plasma glucose, respiration rate (RR), end-tidal CO2 (PetCO 2) and internal carotid artery (ICA) blood flow were recorded at baseline and every 15 min of heat stress. Cognitive function was assessed via the Automated Neuropsychological Assessment Metric at baseline and at 30- and 120 min during heat stress. There were no significant differences between fluid conditions for BM, pOsm, PSI, T c , RR or PetCO 2. Plasma glucose was ∼75% greater in the E + C condition compared to the E condition after 90 min of PHS (P < 0.05). Cognitive function (120 min) was impaired following PHS only in E condition (P < 0.05) and performance on complex cognitive tasks were better by ∼22–340% in the E + C vs. E (P < 0.05). Compared to the E condition, HR and BP were lower and ICA blood flow, vascular conductance, and glucose delivery was ∼90% greater in the E + C after 90 min of PHS (P < 0.05). These data are the first to demonstrate that carbohydrate ingestion may have a protective effect on cognitive function during long duration PHS. Furthermore, this protection was associated with preserved ICA blood flow and glucose delivery to the brain. The primary new findings of the present investigation are as follows: • Prolonged passive heat stress significantly impaired cognitive function as assessed by complex processing tasks. • Exogenous carbohydrate ingestion completely abolished the heat stress-induced impairment in cognitive function. • Exogenous carbohydrate ingestion attenuated the decline in global cerebral blood flow, and thus cerebral glucose delivery was significantly elevated during the bout of prolonged passive heat stress. • Heart rate and mean arterial pressure were lower during heat stress when carbohydrate was ingested, and thus the preserved cerebral blood flow was attributed to an elevated vascular conductance. [ABSTRACT FROM AUTHOR]