The purpose of this study was to detemine the effect of water intake on themoregulatory response during a round of golf (18 holes) in the heat (30-33°C, 55-70% RH) . Ten middle-aged male subjects participated in two separate golf rounds. During the first round the subjects played without fluid replacement (D), while in the second they received water replacement equal to weight loss in D (R) . With D, body weight loss of 3.8% was accompanied by a 13.3% reduction in calculated plasma volume (PV), while a small loss of body weight (0.5%) with an increase in PV of 4.6% was observed in R. A third round of golf by 5 of the subjects in cooler weather (20.4°C) without fluid replacement two months later resulted in heart rate and rectal temperature changes only slightly lower than during R. Golfing with D significantly elevated rectal temperatures and heart rates as compared to R. The rectal temperatures and heart rates at end of the 4.5-hr rounds were 39.4°C and 145 bpm, and 38.6°C and 116 bpm, in D and R, respectively. The serum enzymes, aspaartate aminotransferase, alamine aminotransferase, lactate dehydrogenase, and creatine kinase were measured at rest and after golfing. All enzyme levels increased signijicantly after golfing in D and to a lesser extent in R. The advantages of frequent fluid replacement during golf in high environmental temperatures was clearly demonstrated.