Sex-related differences in heat balance and transfer were studied in nine female and ten male heat-acclimatized subjects exposed to two hot-dry (HD) conditions (49°C, 20% rh; 54°C, 10% rh) and three hot-wet (HW) conditions (32°C, 80% rh; 35°C, 90% rh; 37°C, 80%rh). Exposures lasted 120min: L0 min rest, 50min walk, l0 min rest. 50min walk. Walking speed was 1-34 m s1 (level), and for 49°C, 20% rh, in addition, 1 34ms-1, 5% grade. No sex-related differences were found in metabolic heat production (M), nor in heat exchange by radiation and convection (R + C)-or evaporation (£), when expressed per unit body weight (wt). However, E per unit body surface area (<4D) was lower in females by 9-13% (P <0 05 in all HD conditions and for the 32°C, 80%rh condition) due to their lower M/AD, and 4-6% lower (i? + C)/, 4D in HD. Core-to-periphery heat conductance was similar in both sexes despite a lower core-to-skin temperature gradient for women in HD. It was suggested that women have an advantage over men in heat transfer particularly in H W because of their higher AD/wt. The disadvantage of a high A0/wt at high HD environmental temperatures is diminished by a higher skin temperature, thus reducing (R + C) heat gain. The net effect is to require lower evaporative cooling for women in both HW and HD environments.