[en] For in vivo detection of unstable radical species, a novel radioiodinated radical trapping agent, α-p-iodophenyl-N-tert-butylnitrone (IPBN), was designed based on the plausible characteristics of α-p-hydroxy-m-iodophenyl-N-tert-butylnitrone (HIPBN). Both compounds are iodinated analogs of α-phenyl-N-tert-butylnitrone (PBN), but the latter showed low stability in vivo. The present compound IPBN was easily prepared and labeled with radioiodine. It showed very stable characteristics in vitro as well as in vivo, but was easily metabolized and retained at sites of superoxide production