The purpose of the present study was to examine the effect of water temperature on the human body during low-intensity prolonged swimming. Six male college swimmers participated in this study. The experiments consisted of breast stroke swimming for 120 minutes in 23°C, 28°C and 33°C water at a constant speed of 0.4 m · sec^-1 in a swimming flume. The same subjects walked on a treadmill at a rate of approximately 50% of maximal oxygen uptake (VO₂max) at the same relative intensity as the three swimming trials. Rectal temperature (Tre) in 33°C water was unchanged during swimming for 120 minutes. Tre during treadmill walking increased significantly compared to the three different swimming trials. Tre, mean skin temperature (Tsk) and mean body temperature (Tb) in 23°C and 28°C water decreased significantly more than in both the 33°C water and walking on land. VO₂ during swimming in 23°C water increased more than during swimming in the 28°C and 33°C trials; however, there were no significant differences in VO₂ between the 23°C swimming trial and treadmill walking. Heart rate (HR) during treadmill walking on land increased significantly compared with HR during the three swimming trials. Plasma adrenaline concentration at the end of the treadmill walking was higher than that at the end of each of the three swimming trials. Noradrenaline concentrations at the end of swimming in the 23°C water and treadmill walking were higher than those during the other two swimming trials. Blood lactate concentration during swimming in 23°C water was higher than that during the other two swimming trials and walking on land. These results suggest that the balance of heat loss and heat production is maintained in the warm water temperature. Therefore, a relatively warm water temperature may be desirable when prolonged swimming or other water exercise is performed at low intensity.