MCKEEVER, K. H., W. A. SCHURG, S. H. JARRETT, and V. A. CONVERTINO. Exercise training-induced hypervolemia in the horse.Med. Sci. Sports Exerc., Vol. 19, No. 1, pp. 21–27, 1987. The purpose of this study was to determine if a chronic hypervolemia would accompany endurance exercise training in the horse. Six mature previously inactive horses were utilized for this study. During the 5-wk experiment, five of the horses were trained for 14 d on a treadmill ergometer at a constant treadmill speed of 5.6 km·hr-1and a constant grade of 12.5% for graduated lengths of time. One horse was trained by lunging at a trotting pace in a round pen. Following training, plasma volume increased by 4.7 1 (29.1%,P< 0.05). Although the rate of daily water intake did not change during the training period, 24-h urine output decreased by an average of 3.51· d-1(-24.5%,P< 0.05). Resting glomerular filtration rate and the rate of sodium clearance were not altered by training. However, urea, potassium, and osmotic clearance were decreased by training (P< 0.05) while free water clearance was increased (P< 0.05). Resting plasma aldosterone and arginine vasopressin concentrations were not altered by training. Plasma potassium concentration was significantly decreased (P< 0.05) following the 2 wk of training. These data would appear to suggest that renal control mechanisms affecting water reabsorption via the re-absorption of urea and osmotically active substances other than sodium provide the primary route for the traininginduced hypervolemia seen in horses.