Rapid and pronounced changes in serum iron concentration and leukocyte count in association with open-heart surgery were observed in each of 58 patients. We examined the temporal aspects of these alterations. An initial increase in Fe concentration from a mean of 0.94 mg/L before surgery to 1.20 mg/L was observed within 6 h of the start of surgery. Decreased Fe concentration, a phenomenon previously associated with physiologically stressful events, became apparent 12 h after surgery, by which time the mean Fe concentration had declined to 0.26 mg/L. An increase in the mean leukocyte count, from 7.1 to 15.2 X 1000/mm3 was observed within 6 h of the start of surgery. An increase in ferritin concentration in serum was concurrent with decreased iron concentration.