The aims of the present study were to determine the effects of passive heat exposure on serum lipid concentrations in healthy young Japanese males and to analyze the relationship between subjects' physical characteristics and the extent of change in serum lipid concentrations. Thirteen subjects with mean ages of 22.6 +/- 1.0 (mean +/- SE) years were each exposed to control temperature (Tc: 25.2 +/- 0.0 degrees C), moderate (Tm: 35.5 +/- 0.2 degrees C) and high temperature(Th: 39.8 +/- 0.1 degrees C), at a relative humidity of 60.3 +/- 1.2% for one hour. Each exposure was carried out on a different day in random sequence. Blood samples were collected just before, during and after the exposure, and serum lipid concentrations were analyzed. In the Tc condition, the concentrations of total cholesterol (TC), HDL-cholesterol (HDL-C), LDL-cholesterol (LDL-C), triglyceride (TG), and free fatty acid (FFA) did not change significantly. In the Tm condition, HDL-C increased significantly after the exposure and FFA increased during and after exposure. While in the Th condition, TC and TG decreased significantly during and after exposure, and LDL-C decreased during exposure. The correlation between changes in serum lipid concentrations and physical characteristics was analyzed if lipid concentrations changed significantly during or after exposure, but no significant correlation was found. The results indicate passive heat exposure has an effect on serum lipid concentrations which is independent of physical characteristics.