It has been generally accepted that during exercise at sea level, the pulmonary system of normal, healthy individuals is capable of maintaining arterial oxygen tension at near resting levels. However, recent evidence questions whether this generalization applies to the highly trained endurance athlete who is capable of achieving very high levels of metabolic demand. Hence, the purpose of these experiments was to examine the relationship between maximal oxygen consumption (VO2max) and arterial oxygen-hemoglobin saturation (%SaO2) during short-term heavy exercise in trained athletes and untrained individuals. Ten trained distance runners and 7 untrained males exercised at 95% of VO2max for 3 min. Minute-by-minute measurement of %SaO2 was obtained via ear oximetry. The correlation coefficients between %SaO2 and VO2max during exercise were r = -0.68, r = -0.74, and r = -0.72 (P less than 0.05) for minutes 1 through 3, respectively. In general those individuals with the highest VO2max showed the greatest decrease in %SaO2. By comparison there was no difference (P greater than 0.05) in resting %SaO2 between the trained (96.3 +/- 0.2% [SE]) and the untrained (96.3 +/- 0.4%) subjects. However, at minute 3 of exercise, %SaO2 was significantly lower (P less than 0.05) in the trained subjects (87.0 +/- 0.7%) than in the untrained subjects (92.6 +/- 0.7%). These data demonstrate that arterial desaturation occurs in healthy, highly trained endurance athletes during heavy exercise and that the level of the arterial desaturation is inversely related to VO2max.