The in vivo relationship between blood T2* and oxygen saturation was investigated. A wide range of blood oxygen saturation levels was created in pigs by altering the ventilation rate. Blood T2* was measured in vivo in the abdominal aorta and the inferior vena cava, corresponding to each oxygen saturation level. Our results indicate that it is possible to measure blood T2* in vivo reliably with an MR technique we previously proposed. Blood T2* correlates closely with oxygen saturation levels over a wide range, and the relationship between blood T2* measured in vivo and oxygen saturation can be approximated by a linear function.