In order to evaluate metabolic heat generation and heat dissipation of strenuously exercising horses, thermal response in various body compartments was simultaneously and continuously monitored. Two horses were subjected to a long-term incremental exercise protocol with progressive increase of treadmill speed, slope and draught load. The temperatures were determined in the following structures and cavities using implanted thermocouple probes: middle gluteal, semitendinosus and masseter muscles, right cardiac ventricle, jugular vein, abdomen, rectum and subcutaneous tissue. The kinetics of temperature increases were similar for locomotor muscles, right ventricle (body core), abdomen and subcutaneous tissue. At peak exercise the highest temperatures were measured in locomotor muscle (43.3 degrees C). The blood temperature in the jugular vein was approximate to 1 degrees C below body core temperature at rest and approximate to 3 degrees C below at peak exercise. During and after exercise the rectal temperature never accurately assessed core and compartment temperatures. Maximal rectal temperatures peaked approximate to 1 degrees C below body core temperature with a lag time of 10-15 minutes.