The diffusing capacity of the lung for carbon monoxide (DLCO) decreases to below the pre-exercise value in the hours following a bout of intense exercise. Two mechanisms have been proposed: (1) development of pulmonary oedema and (2) redistribution of central blood volume to peripheral muscles causing a reduction in pulmonary capillary blood volume (V c). In the present study DLCO, V c and the membrane diffusing capacity (D m) were measured in nine healthy females using a rebreathing method, in contrast to the single breath technique employed in previous studies. DLCO, V c and D m were measured before and at 1, 2, 3, 16 and 24 h following maximal treadmill exercise. Compared with pre-exercise values, DLCO was depressed by up to 8.9 (3.0)% (P<0.05) for the first 3 h following exercise, but had returned to pre-exercise values by 16 h post-exercise. V c fell by 21.2 (4.1)% (P<0.05) at 3 h post-exercise, but at the same time D m increased by 14.7 (9.1)%. It was concluded that: (1) the increase in D m made it unlikely that the fall in DLCO was due to interstitial oedema and injury to the blood gas barrier; (2) on the other hand, the reduction in DLCO following exercise was consistent with a redistribution of blood away from the lungs; and (3) the trend for D m and V c to reciprocate one another indicates a situation in which a fall in V c nevertheless promotes gas transfer at the respiratory membrane. It is suggested that this effect is brought about by the reorientation of red blood cells within the pulmonary capillaries following exercise.