Inhaled oxygen can be used as a contrast agent for magnetic resonance imaging, due to the T1 shortening effect of the oxy- gen dissolved in blood and tissue water. In this study, blood T1 was measured dynamically in 14 volunteers (seven smok- ers, 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 pres- sure of oxygen, which were found to be in agreement with lit- erature 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 differ- ences between groups highlight the importance of measuring such an input function for each individual rather than relying on an assumed measurement.