Six intact and 6 splenectomised mares were given an incremental exercise test on a treadmill to examine the fluid and electrolyte changes associated with exercise and the role of the spleen in these changes. Blood samples were obtained at rest and at the end of each 1-min step of the test. Exercise at 7 m/sec caused significant (P < 0.05) increases in plasma osmolality (intact, +9.9%; splenectomised, +6.2%), plasma protein concentration (intact, +15.8%; splenectomised, +11.4%), and plasma K+ concentration (intact 46%; splenectomised, +57%). Plasma Na+ concentration did not increase significantly (P > 0.05) in either group (intact = 2.7%; splenectomised, = 3.5%). This appears to be the first record of substantial changes of these constituents during short-term exercise, even before the onset of visible sweat losses. The changes in the concentration of plasma protein suggest that short-term exercise causes a decrease in plasma volume. The changes in the concentration and content of sodium suggest that this is an isotonic shift of fluid. The increase in plasma potassium concentration appears to be due to haemoconcentration as well as an increase in the content of potassium within the vascular compartment.