The effects of acute fluid loss on renal concentrating function was measured in young athletes. Weights, bloods and urines on five athletes were evaluated before and after (a) basketball practice producing a 4-lb. weight loss and a slight rise in serum osmolality, (b) exercise plus exogenous ADH, (c) sauna bath in which a 4-lb. weight loss per athlete was sustained, and (d) sauna bath following water load. In each of these experimental situations urine osmolality either did not change or decreased slightly. During the sauna bath the urine osmolality, already depressed by the water load, changed little as the creatinine clearance, osmolar clearance, and fractional excretion of sodium decreased markedly. Over the next 2 h at rest at room temperature, the urine osmolality and creatinine clearance increased while the fractional excretion of sodium remained depressed. It is concluded that in acute fluid loss induced by exercise or sauna bath in normal subjects, there is a delay in enhancement of urine osmolality at a time when the glomerular filtration rate is greatly depressed.