The purpose of this study were to investigate the bed climate and the physiological responses in sleep on the traditional Korean floor heating system called Ondol. Ondol has been the most widely used heating system in Korea, yet there has been a no systematic studies examined its environmental effects on human body. Experimental room was constructed to match the typical thermal environment of Ondol. (floor surface temperature; $32~33^{circ}C$, air temperature; $22.5{pm}1.0^{circ}C$, relative humidity; $64.0{pm}4.0\%$RH, air velocity; 0.25 m/sec.) Three different combinations of bedclothes were chosen for the experiment based on the study results showing that they were the most widely used types in Korea. Type 1 was the combination of a cotton-padded mattress with a cotton-padded Korean style blanket. Type 2 was a cotton-padded mattress with a cotton-guilted Korean style blanket. Type 3 was a cotton-padded matless with a polyacryl blanket. Thermal resistance of each of these combina-tions in the bedclothes was measured using thermal manikin. Two adult female was chosen for the seven hour sleeping experiment which was known to be the average sleeping hours of Korean adult female. The bed climate was measured with the temperature under the mattress, the surface temper-ature of the mattress, and the air temperature and the relative humidity of the space between the mattress and blanket. The skin temperature, rectal temperature of the subjects and the bed climate were measured eight times, one hour before the experiment and every hour during the experiment. The weight loss and the subjective sensation were measured for the each subjects before and after the experiment. The procedure was repeated twice with two subjects and three types of bedclothes, yielding twelve combinations of results. The results were as follows; 1. With the surface temperature of $32~33^{circ}C$ of Ondol, air temperature was $22.5{pm}1.0^{circ}C$ with $64.0{pm}4.0\%$ RH. The bed climates were $39.2{pm}40.8^{circ}C$ under the mattress, $35.3~36.2^{circ}C$ on the mattress, and $26.9~32.0^{circ}C$ with $56.0~71.3\%$ RH between the mattress and the blanket. 2. Mean skim temperature during sleep was 34.2"C with local skin temperature of $34.0~35.5^{circ}C$. The skin temperatures of abdomen, thigh, foot were higher than the other parts of the body. 3. The skin temperature of chest, thigh, leg and back varied significantly according to the combinations of bedclothes. With the cotton-padded blanket, the skin temperature was the highest, while with the cotton-guilted blanket showed lowest. 4. Examining the relationship between the mean skin temperature and the local skin temper-ature, the chest temperature showed the highest correlation with the former. Therefore, the chest temperature can be recommended to represent the skin temperature in measuring the bed climate. 5. The subjective bed climates were $39.0~40.4^{circ}C$ under the mattress, $35.2~35.9^{circ}C$ on the mattress, $29.8~31.6^{circ}C$ with $56.8~68.4\%$ RH between the mattress and blanket. In sum, from this experiment we not only obtained the reliable value of bed climates on Ondol, but also showed that the bed climates and the physiological responses were affected differently according to the materials of bed clothes.