Application of the spin trapping technique in intact animals requires an understanding of the stability and distribution of the spin traps and their spin adducts in vivo. We studied the stability of DEPMPO and its spin adduct DEPMPO/SO3- in living mice. We found that DEPMPO is approximately evenly distributed in vivo, as tested in mice. About 40% of the DEPMPO remained in the liver, heart and blood of mice 60 min after injection. DEPMPO did not have any toxic effect in cultured cells up to the concentration of 25 mM. At 50 mM, however, very significant toxicity was observed. Using SO3- as our model system, we were able to observe the spin adduct DEPMPO/SO3-directly from a living mouse, with a DEPMPO concentration as low as 2 mM. The spin adduct DEPMPO/SO3- is several times more stable than DMPO/SO3- in vivo. Based on these results as well as reports from other laboratories, we conclude that DEPMPO is a potentially a good choice for many spin trapping applications in biological systems.