Effects of sleep deprivation and season on thermoregulation during 60 min. of leg-bathing (water temperature of 42°C, air temperature of 30°C, and relative humidity of 70%) were studied in eight men who completed all 4 experiments for normal sleep and sleep deprivation in summer and winter. Rectal temperature ( T _re), skin temperature, total body sweating rate ( M _sw-t), local sweating rate on the back ( M _sw-back) and forearm ( M _sw-forearm), and skin blood flow on the back ( SBF _back) and forearm ( SBF _forearm) were measured. The changes in T _re (Δ T _re) were smaller ( P <0.05) for sleep deprivation than for normal sleep regardless of the season. This decrease in Δ T _re was significant only in summer ( P <0.05). Mean skin temperature ( T (mean of)_sk) was higher ( P <0.05) for sleep deprivation than for normal sleep regardless of the season. M _sw-t was smaller ( P <0.05) for sleep deprivation than for normal sleep regardless of season, although M _sw-back and M _sw-forearm were similar. S BF _back and SBF _forearm tended to be higher for sleep deprivation than normal sleep. The sensitivity of SBF to T _re was higher ( P <0.05) for sleep deprivation than for normal sleep. These data indicate that seasonal differences in thermoregulation were small because of morning time. Sleep deprivation increased dry heat loss and restrained T _re rise, in spite of decreased sweating rate.