Nine adult riding horses performed on a treadmill a competition exercise test (CET) and 24 h later a standardised exercise test (SET) at mean temperature 28 degrees C and relative humidity 58%. Each horse performed the tests 5 times at 2 week intervals. The horses were given isotonic glucose-electrolyte solution via a nasogastric tube 30 min after the CET, except after the last trial when water was given. Blood lactate, plasma concentration of aldosterone, arginine vasopressin, protein, sodium, potassium, chloride, magnesium and calcium were measured. Heart rate at blood lactate concentration 4 mmol/l (PLa4) in the SET, plasma volume (PV) and red cell volume (CV) were determined. Mean weight loss after CET was 3.1% and did not change significantly during the study. Bodyweight loss (BWL2) before SET (-1.8% after the first trial) decreased linearly with time (P < 0.05) and was -0.8% after the fourth trial. After the fifth trial, when only water was given, the weight loss was -2.5%. PLa4 correlated significantly (P < 0.001) with BWL2 when BWL2 was greater than -1.2%. The horses acclimated to exercise in hot and humid conditions as indicated by better recovery of bodyweight, increased PV, lower peak lactate concentrations and heart rate and attenuated decrease in the concentrations of sodium and chloride. It is concluded that changes in bodyweight provide a good indication of recovery of horses after exercise in hot and humid conditions; and administration of an isotonic glucose-electrolyte rehydration solution after exercise helps to overcome dehydration better than water alone.