Dihydromethidine (DHM) labeled with ¹⁸F at the para position of the peripheral benzene ring was designed as a positron emission tomography (PET) radiotracer for non-invasive imaging of reactive oxygen species (ROS). This compound readily crosses the blood–brain barrier and is oxidized by ROS, and the oxidation product is retained intracellularly. PET imaging of ROS-producing rat brain microinfused with sodium nitroprusside identified specific brain regions with high ROS concentrations. This tracer should be useful for studies of the pathophysiological roles of ROS, and in the diagnosis of neurodegenerative diseases.