Total body water (TBW) and water turnover rates (WTR) of six competitive male cyclists (CG) and six age-matched sedentary men (SG) were determined using deuterium oxide dilution and elimination. During the 7 day study, individuals in the CG cycled daily outside on average 50 (range 12-146) km at an average speed of 29 km.h(-1), while the SG did no regular exercise. During the study, the weather was cool (10 [4-18]degrees C), mainly cloudy but dry. Daily average (median [range]) nude body mass remained essentially the same in the CG (77.25 [76.54-77.54] kg) and SG (65.04 [64.45-65.44] kg). Expressed as a percentage of body mass, median TBW of the CG (70.1 [65.5-73.9]%) was greater than that of the SG (63.5 [52.7-71.0]% ). Average median WTR was faster in the CG (47 [42-58] ml.kg.d(-1)) than the SG (36 [29-50] ml.kg.d(-1)). The average median daily urinary loss was similar in the CG (27 [22-33]ml.kg.d(-1)) and SG (29 [24-31]ml.kg.d(-1)). Calculated non-renal daily water loss was faster in the CG (19 [13-35] ml.kg.d(-1)) than the SG (6 [5-22] ml.kg.d(-1)), but there was no relationship between the average distance cycled daily and the WTR. This study demonstrates that WTR are faster in individuals undertaking prolonged exercise than in sedentary men, and that the difference was due to the almost three times greater non-renal water losses that the exercising group incurred. This suggests that exercise-induced increases in respiratory water loss and sweat rate are major factors in water loss even in cool environments.