It is well known that exercise training in a hot environment (EH) induces significant physiological changes in humans, including increased plasma volume, dropped core temperature, and enhancement of thermoregulatory responses such as sweating and cutaneous vasodilation. During passive heating in resting humans, ventilation increased and cerebral blood flow decreases, but the effect of EH on ventilatory and cerebral circulatory responses during passive heating remains to be seen. Further, it is also unclear whether ventilatory and cerebral circulatory responses during passive heating are changed after exercise training in cold environment (EC) (heat stress during training is attenuated). PURPOSE: To investigate the effects of exercise training in cold and hot environment on ventilation and cerebral circulation during passive heating at rest in humans. METHODS: Subjects performed passive-heat tests before and after EC (n = 10) and EH (n = 10). During the training period, subjects performed 4 bouts of 20-min exercise at 50% peak oxygen uptake (VO2peak) separated by 10-min rest in cold (EC, 10 °C) and in hot (EH, 37 °C) environment for 6 days. In the passive-heat tests, hot-water legs-only immersion and a water-perfused suit were used for heating. RESULTS: Core temperature evaluated by esophageal temperature (Tes) during training was higher in EH than in EC (38.6-38.7 vs. 37.8-38.0 °C). VO2peak was increased by 5-6% following both EC and EH, and resting plasma volume was increased by 7% only after EH. Tes during passive-heat test was lowered only after EH (∼0.4°C). Minute ventilation during passive-heat test became higher after EC (9 l/min), while it was not changed after EH. Cerebral vascular conductance index, evaluated from middle cerebral artery mean blood velocity measured by transcranial doppler ultrasound divided by mean arterial blood pressure (MAP), was higher during passive-heat test only after EH (∼6%). Tes threshold for increase in VE and slopes relating ventilatory variables and Tes did not change after both EC and EH. Tes threshold for increase in forearm vascular conductance, evaluated from forearm blood flow measured by venous occlusion plethysmography divided by MAP, was lowered only after EH (0.3°C). CONCLUSIONS: These results suggest that in humans, 1) following exercise training in a hot environment, core temperature is lowered, cerebral vasoconstriction is attenuated, and cutaneous vasodilation response known as thermoregulatory response is enhanced during passive heating, while 2) following exercise training in a cold environment, these changes are not induced and ventilatory response during passive heating is enhanced.