The serum concentrations of serum amyloid A, haptoglobin and fibrinogen were measured in a group of horses before and at intervals after elective and non-elective surgery, and in a control group of normal horses. There was a significant, rapid and repeatable increase in the concentration of serum amyloid A in response to both elective and non-elective surgery. In the control horses its serum concentration was within the normal range, from 0 to 0·2 µg/ml. Twenty-four hours after elective surgery its mean peak concentration was 16·4 µg/ml, and after non-elective surgery it was 27·3 µg/ml. In contrast, the serum concentrations of haptoglobin and fibrinogen increased more slowly after surgery and had not decreased by 72 hours after surgery.