Cerebral blood flow (CBF) is reduced during passive heat stress, with 50% of this reduction associated with hyperventilatory-induced hypocapnia and subsequent cerebral vasoconstriction. It remains unknown, however, what other factors may contribute to the remaining 50%. We tested the hypothesis that the distribution of cardiac output plays an important role in maintaining cerebral perfusion during mild and severe heat stress. Middle cerebral artery and posterior cerebral artery blood flow velocity (MCAv and PCAv; transcranial Doppler) and left ventricular end-diastolic and end-systolic volumes (2-D echocardiography) were measured under conditions of normothermia and mild and severe passive heat stress (core temperature +0.8 ± 0.1°C (Protocol I; n= 10) and 1.8 ± 0.1°C (Protocol II; n= 8) above baseline). Venous return was manipulated by passive tilt table positioning (30 deg head-down tilt (HDT) and 30 deg head-up tilt (HUT)). Measurements were made under poikilocapnic and isocapnic conditions. Protocol I consisted of mild heat stress which resulted in small reductions in end-tidal CO 2 (-5.6 ± 3.5%), MCAv/PCAv (-7.3 ± 2.3% and -10.3 ± 2.9%, respectively) and stroke volume (-8.5 ± 4.2%); while end-diastolic volume was significantly reduced (-16.9 ± 4.0%) and cardiac output augmented (17.2 ± 7.4%). During mild heat stress, CBF was related to left ventricular end-diastolic volume (MCAv, r 2= 0.81; PCAv, r 2= 0.83; P < 0.05) and stroke volume (MCAv, r 2= 0.38; PCAv, r 2= 0.43), but not with cardiac output. Protocol II consisted of severe heat stress which resulted in much greater reductions in end-tidal CO 2 (-87.5 ± 31.5%) and CBF (MCAv, -36.4 ± 6.1%; PCAv, -30.1 ± 4.8%; P < 0.01 for all variables), while end-diastolic volume and stroke volume decreased to a similar extent as for mild heat stress. Importantly, isocapnia restored MCAv and PCAv back to normothermic baseline. This investigation therefore produced two novel findings: first, that venous return and stroke volume are related to CBF during mild heat stress; and second, that hyperventilatory hypocapnia has a major influence on CBF during severe passive heat stress. © 2011 The Authors. Journal compilation © 2011 The Physiological Society.