AbstractInhaled oxygen can be used as a contrast agent for magnetic resonance imaging, due to the T1 shortening effect of the oxygen dissolved in blood and tissue water. In this study, blood T1 was measured dynamically in 14 volunteers (seven smokers, seven never‐smokers) as the inhaled gas was switched from medical air to 100% oxygen and back to medical air. These T1 values were converted to changes in partial pressure of oxygen, which were found to be in agreement with literature values. There were differences in curve shape and curve height between the smoker and never‐smoker groups, suggesting differences in lung function due to smoking‐related damage. These curves could be used as an input function for modeling of oxygen uptake in tissues. The differences between groups highlight the importance of measuring such an input function for each individual rather than relying on an assumed measurement. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.