It has been known since the 1920s that runners completing marathon races have reduced forced vital capacity (FVC) values. To investigate the time course of these lung volume alterations, we measured FVC and residual volume (RV) in 11 runners before, after, and at 30-min intervals during a 2.5-h treadmill run at just under their marathon pace (70% VO2max). Mean distance run was 21.5 +/- 1.5 (SD) miles in the 2.5-h period. During the first 60 min, both RV and total lung capacity (TLC) decreased by 110 ml, however, this change was not significant (P greater than 0.05). A high correlation (r = 0.93) was observed between delta RV and delta TLC during the first 5 min, while FVC remained unchanged. From 60-90 min, all lung volumes remained constant. From 90-150 min, lung volumes changed in a direction similar to that observed after a marathon, i.e., FVC decreased significantly (5.51 to 5.37 liter between 90 and 150 min, P less than 0.05), TLC remained unchanged (7.41 vs 7.42 liter, P greater than 0.05), and RV showed a nonsignificant increase from 1.90 to 2.05 liter (P greater than 0.05). The data are consistent with multiple mechanisms playing a role in pulmonary function changes during prolonged exercise. The smaller mean decrease in FVC observed in this study, as compared to that found during a marathon, suggested that the marathon imposes a greater demand on the lungs than did treadmill exercise of the duration and relative intensity used in this study.