The concentrations of plasma fibrinogen and serum haptoglobin and the white blood cell count (WBC) were assessed in 17 thoroughbreds. All horses were trained for 80 days, six days a week, with one day of rest per week. Blood samples were collected from each animal by jugular venepuncture, at 06.00, every 20 days for a period of 80 days, and the fibrinogen and haptoglobin concentrations and WBC were determined. One-way repeated measures analysis of variance, followed by the Bonferroni test, was used to determine significant differences. Training was associated with a significant increase in the concentrations of plasma fibrinogen (P<0.01) and serum haptoglobin (P<0.0005), but had no significant effect on the WBC count.