Derivatives of N-hydroxyurea that contain an N-hydroxy group react with oxyhemoglobin to form methemoglobin and variable amounts of nitrite/nitrate. Compounds with an unsubstituted -NHOH group produce the most nitrite/nitrate, which provides evidence for nitric oxide formation. The rate of reaction of these N-hydroxyurea derivatives with oxyhemoglobin correlates well with that compound's oxidation potential. Aromatic N-hydroxyureas react 25-80-fold faster with oxyhemoglobin than with N-hydroxyurea, suggesting other N-hydroxyurea analogues may be superior nitric oxide donors. Electron paramagnetic resonance spectroscopy shows that the formation of a low-spin methemoglobin-hydroxyurea complex is critical for iron nitrosyl hemoglobin formation. These results show that iron nitrosyl hemoglobin formation from the reaction of hydroxyureas and hemoglobin requires an unsubstituted -NHOH group and that the nitrogen atom of the non-N-hydroxy group must contain at least a single hydrogen atom. These results should guide the development of new hydroxyurea-based nitric oxide donors and sickle cell disease therapies.