Thermal stress affects key modulators of cerebral blood flow (CBF), in particular arterial pressure and arterial carbon dioxide content (PCO2) as a result of blood redistribution and hyperventilation. There are conflicting findings regarding whether heat stress alters the acute cerebrovascular responsiveness to CO2 (CBV-CO2 responsiveness). Methodological differences in how the heat stress is induced (passive vs. exercise-induced) and the order in which the hypercapnic (i.e., increased CO2) and hypocapnic (i.e., decreased CO2) challenges are administered may explain these differences. The effect of cold stress on CBV-CO2 responsiveness has not been explored. Changes in CBV-CO2 responsiveness during thermal stress could affect CBF functional reserve in extreme environments and increase the risk of brain hypo- and/or hyper-perfusion-related events (e.g., fainting, stroke). Therefore, the purpose of this study was to examine the effect of passive heat and cold stress on CBV-CO2 responsiveness. Eight participants dressed in a water perfusion suit and performed hypercapnic (5% CO2 inspiration) and hypocapnic (voluntary hyperventilation) CO2 responsiveness tests under heat (increase in core temperature of 1.3±0.3°C (mean±SD) and cold stress (to shivering onset) conditions. Thermal stress tests were randomised-counterbalanced and proceeded by normothermic control. To evaluate CBV-CO2 responsiveness the slope of the relation between middle cerebral artery blood velocity (MCAv) and PETCO2, and cerebrovascular conductance (CVC; MCAv/mean arterial pressure (MAP)) and PETCO2 was calculated. Interaction and main effects were determined using repeated-measures ANOVA. Heat stress decreased MCAv by 14±14 cm/s (P=0.03), CVC by 0.15±0.14 cm/s/mmHg (P=0.02) and PETCO2 by 4±3 mmHg (P<0.01), while MAP remained similar (80±6 vs. 82±7 mmHg; P=0.51) at baseline, compared to normothermia. Cold stress did not change MCAv (74±19 vs. 76±17 cm/s; P=0.66) or PETCO2 (37±3 vs. 38±3 mmHg; P=0.58), while CVC decreased by 0.15±0.14 cm/s/mmHg (P=0.02) and MAP increased by 14±8 mmHg (P<0.01) at baseline, compared to normothermia. The slope of the hypercapnic MCAv-CO2 responsiveness relation decreased during heat stress (P=0.04 vs. normothermia) and increased during cold stress (P<0.01 vs. normothermia; heat vs. cold, P<0.01), but was not significantly different to hypocapnic MCAv-CO2 responsiveness for any condition (P>0.07). The slope of the CVC-CO2 responsiveness relation was unchanged across thermal conditions (P=0.93), but was greater in hypocapnia than hypercapnia (P<0.01). These results indicate that thermoregulatory mechanisms influence MCAv-CO2 responsiveness; primarily due to changes in blood flow redistribution, as calculation of CVC-CO2 responsiveness removed such effects and MCAv-CO2 responsiveness was deferentially affected by the contrasting haemodynamic responses elicited by heat and cold stress.