The effects of exercise training on hepatic function in horses were determined by studying the plasma clearance of antipyrine (20 mg/kg iv) in adult mares that either underwent treadmill training for 5 wk ( n = 7) or remained in box stalls for the same time period ( n = 6). Training consisted of treadmill exercise at 60% (12 min/day) and 90% (3 min/day) of pretraining maximal oxygen consumption (V˙o 2 max) for 6 days/wk for 5 wk.V˙o 2 max and velocity to obtain a blood lactate concentration of 4 mmol/l were significantly increased (from 129 to 149 ml ⋅ min−1 ⋅ kg−1and from 5.6 to 6.1 m/s, respectively) as a result of training. The plasma clearance and volume of distribution of antipyrine increased significantly in the trained group (from 5.5 to 6.4 ml ⋅ min−1 ⋅ kg−1and from 813 to 881 ml/kg, respectively) and decreased significantly in the untrained group. Elimination half-lives did not change as a result of training or box rest. Increases in plasma antipyrine clearance were indicative of an increase in hepatic metabolism of antipyrine. Increases in the volume of distribution of antipyrine suggest that total body water increases as a result of exercise training.