To investigate relationships between pulmonary transit times (PTT) and pulmonary diffusion limitation during exercise, 10 high aerobic capacity athletes (VO2max = 5.15 +/- 0.52 l.min-1) who had multiple inert gas elimination analysis evidence suggestive of diffusion disequilibrium were studied at rest and maximal exercise. Diffusing capacity for oxygen (DLO2) was calculated from the inert gas data. First pass radionuclide angiography was performed using 99mTechnecium labeled erythrocytes and whole lung PTT and pulmonary blood volume (PBV) were calculated. PTT decreased from 9.32 +/- 1.41 sec at rest, to 2.91 +/- 0.30 sec during exercise and was correlated with diffusion limitation suggested by the inert gases (r = -0.58, P < 0.05). PBV increased during exercise to over 25% of whole blood volume and correlated with DLO2 (r = 0.82, P < 0.01). These data suggest that diffusion limitation is related to shortened PTT in athletes and that maximal recruitment of PBV may defend against diffusion limitation.