In pig production, heat stress can be extremely detrimental to welfare and production over a range of thermal conditions. The results of heat stress may range from decreased feed intake and growth efficiency, reproduction and even hyperthermic death. In order to alleviate heat stress, many cooling systems for pig housing have been developed in recent years (Gates et al., 1991; Panagakis et al., 1996; Eigenberg et al., 2002). In China, the approaches such as the fan and pad cooling system (mostly used for growing/finishing pigs), the drip cooling system (mostly used for gestating sows), and the underground tunnel air-cooling system has been used (Liu et al., 1997; Dong et al., 1998, 2001; Ma, 1997). Those systems were mainly designed and applied in confined pig production buildings. Although the indoor environment is improved, common problems of the cooling systems during operation arise, such as increasing humidity, wet and dirty floor, noise level, etc., which will damage pig health and decrease pig production. Many of those cooling systems cannot be used in an open type pig house. Pig production systems with open structures and free access to an outdoors area are very common in China. The thermal conditions in these types of pig housing are very much dependent on the weather. In hot weather, feasible and effective cooling systems will be in great demand to provide a comfortable environment for the pigs. Generally, on hot days pigs may cool themselves by wallowing or enjoying water sprinklers (Heitman et al., 1962), and they may seek protection from the sun in the shade (Heitman et al., 1962; Blackshaw & Blackshaw, 1994) if shading facilities are available. They may also attempt to increase their heat loss by moving away from hot places to a cooler area or to a place with higher air velocity, by changing their lying posture from the belly to the side, or by avoiding having body contacts with other pigs (Geers et al., 1986). By rolling from side to side in a wallow or a damp place, the pigs may benefit from the evaporative heat loss via cooling their moist upper skin and conductive heat loss via contact in water. Since pigs spend more time resting than any other domestic animals, i.e. about 80% of the time daily (Haugse et al., 1965), relaxation and sleep are very important for pig health and growth rate. Therefore, a cooling system to meet the lying demands of pigs in an open type house under hot weather conditions is essential for the optimal production as well as for the welfare of pigs. Experiments were conducted to compare the floor temperatures and to observe the lying behaviour of pigs in the sleeping area of the buildings with and without floor cooling system. The results showed that, without the floor cooling system, the floor temperature was nearly the same as the air temperature in the open pig house. With the floor cooling system, the floor temperature of the sleeping area was controlled at 22–26 °C, even though the air temperature was as high as 34°C, which improved the comfort of the pigs in the sleeping area. The pig lying behaviour was greatly affected by the floor temperature. More than 85% of the pigs were lying in the sleeping area when the floor temperature was below 26 °C, while only 10–20% of the pigs were lying in the sleeping area when the floor temperature was about 30 °C, and hardly any when the floor temperature was above 33 °C.