The effect of external heat-load, exercise and dehydration on dynamic changes in plasma cortisol during the development of heatstroke was investigated. Thirty-three unanesthetized dogs were tested under two sets of climatic conditions: comfort conditions and hot-dry climatic conditions, half of them while exercising. Half of the dogs in each group were rehydrated. None of the dogs that were investigated at room temperature suffered heatstroke. Of the dogs exposed to high ambient temperature, all of the exercising, as well as five out of six non-hydrated dogs and one rehydrated non-exercising dog suffered heatstroke. Significant dehydration (6%-7% of body weight), occurred only under high ambient temperature. Plasma cortisol levels of all dogs that suffered heatstroke rose conspicuously for at least 5 h and returned to normal levels 24 h later. Cortisol levels of dogs who did not experience heatstroke remained within the normal range. Cortisol levels correlated with the severity of the stress leading to heatstroke. High and rising levels of cortisol, several hours after body temperature returns to normal, may support the diagnosis of heatstroke.