Five Chilean crossbred draught horses were used to study variation in work performance and biochemical and physiological parameters whilst working with a mould board plough (36 kg) for 6 h/day. The draught force, distance travelled, work done and the estimated extra energy for work were determined for each horse. The results showed that the horses used only 68·0 (s.e. 1·9)% of the total available working time ploughing a mean area of 3283 (s.e. 168) m2 with a mean depth and width of furrow of 12·6 and 22·3 cm, respectively. Estimated daily energy expenditure by the horses during work was 2·24 (s.e. 0·02) times maintenance requirements. The small changes observed in the physiological variables: heart rate, respiratory rate, packed cell volume, haemoglobin concentration and enzyme activity of creatine kinase, lactate dehydrogenase and aspartate amino transferase, showed that the horses were exercised under submaximal conditions. However, the significant increase in blood cortisol concentrations showed that horses experienced some degree of stress during ploughing work. The significant increase in blood triglycerides concentration observed after each period of work seems to demonstrate that fat mobilization is an important metabolic pathway as an energy supply for the working muscles during prolonged and low speed draught work.