Although men exposed to cold maintain a significantly warmer skin at altidude than at sea level, their core temperature is not lower. To gain more information on the effect of altitude on heat loss processes, skin (T(sk)) and rectal (T(re)) temperatures, sweat gland activity (sga) and other related variables were measured at sea level and 4,360 m in 4 lowlanders (LOL) and at 4,360 m in 4 highlanders (HIL) during a 2-hr rest and a 30-min exercise in T(a) 35 °C, rh 40%, and low air movement; a 1-hr control period in T(a) 25°C preceded the heat exposure. Both the T(sk) and T(re) of the LOL stabilized in 35°C at significantly higher values during rest at altitude than at sea level; during exercise at sea level, their T(sk) fell and their T(re) rose, but at 4,360 m both increased; sga was significantly greater at altitude than at sea level, during both rest and exercise. During rest, the T(sk) and T(re) of the HIL increased in the heat significantly less than those of the LOL at altitude; during exercise, their T(sk) also stabilized at a significantly lower value than that of the LOL, but their T(re) was not different; their sga was greater at rest than that of the LOL, but not different during exercise. Given previous findings that altitude reduces cutaneous blood flow, the present higher T(sk) and sga at altitude as compared to sea level suggest that altitude exposure may hinder convective heat transfer while promoting sweating.