Leukocytosis was postulated to accompany short- and medium-length exercise; in this report, we have studied the changes in leukocyte count during and following prolonged exercise. White blood cell (WBC) counts were obtained in 15 endurance-trained subjects before, during, and at a recovery period after an ultralong exercise (120 km march), lasting 24 h. WBC counts increased after 16 h march from a baseline value of 8.5 +/- 0.3 10(9) l-1 to 11.3 +/- 0.8 10(9) l-1 (P less than 0.05) and then declined to 7.1 +/- 0.9 10(9) l-1 after 24 h march with no further significant changes during 64 h of recovery. These observations were supported by previous findings in three separate marches performed by a second group (40, 70, and 120 km). A parallel increase in plasma creatine phosphokinase activity from 127 +/- 4.4 ul-1 to 539 +/- 106.3 ul-1 was observed after 16 h march (P less than 0.01), indicating muscle cell damage. Our findings suggest that in extremely long marches, WBC counts return to baseline values before exercise is terminated. This phenomenon may reflect WBC infiltration to damaged muscle tissue.