1. The response of circulating leucocytes with regard to changes in number, proportion of granulocytes and lymphocytes, as well as changes in the ascorbic acid (AA) concentration of plasma and isolated lymphocytes, were studied in nine men who ran a 21 km race. A marked leucocytosis was noted 5 min after the race, the predominant increase being in granulocytes (P less than 0.001) with smaller relative increases in circulating lymphocytes (P less than 0.01) and platelets (P less than 0.001). Numbers of leucocytes and platelets returned to pre-exercise levels within 24 h after the race. 2. The concentration of AA in plasma increased from 52.7 +/- 4.1 mumol/l before the race to 67.0 +/- 5.3 mumol/l within 5 min after the race (P less than 0.001). This increase in plasma AA concentration was positively correlated with the rise in plasma cortisol concentration during the race (r = 0.89; P less than 0.01). However, within 24 h after the race the plasma concentration of AA fell 20 +/- 4% below pre-exercise values (P less than 0.01) and remained low for at least the next 2 days (P less than 0.05). 3. Lymphocyte AA concentration increased from 15.6 +/- 0.6 to 19.7 +/- 0.9 mumol/g of lymphocyte protein during the race (P less than 0.01) but returned to normal levels within 2 days after the race. 4. It is suggested that the adrenal gland may be the major source of AA efflux into the circulation during exercise.