We investigated whether heat-induced hyperventilation during exercise is affected by time-of-day. Nineteen male subjects were divided into two experiments (protocol 1, n=10 and protocol 2, n=9). In protocol 1, subjects performed cycle exercise at 50% peak oxygen uptake in the heat (37°C and 50% RH) in the morning (06:00) and evening (18:00). Results showed that baseline resting and exercising esophageal temperature (Tes) were significantly (0.5°C) higher in the evening than morning. Minute ventilation (VE) increased from 54.3±7.9 and 54.9±6.8 l•min-1 at 10 min to 71.4±8.1 and 76.5±11.8 l•min-1 at 48.5 min in the morning and evening, respectively (both P<0.01). Time-of-day had no effect on VE (P=0.44). When VE as the output response was plotted against Tes as thermal input, the Tes threshold for increases in VE was higher in the evening than morning (37.2±0.7 vs. 36.6±0.6°C, P=0.009), indicating the ventilatory response to the same core temperature is smaller in the evening. In protocol 2, the circadian rhythm-related higher resting Tes seen in the evening was adjusted down to the same temperature seen in the morning by immersing the subject in cold-water. Importantly, time-course of changes in VE during exercise were smaller in the evening, but the threshold for VE remained higher in the evening than morning (P<0.001). Collectively, those results suggest that time-of-day has no effect on time-course of hyperventilation during exercise in the heat, despite higher core temperatures in the evening. This is likely due to diurnal variation in the control of ventilation in response to rising core temperature.