The anaerobic formation of nitric oxide haemoglobin (NOHb) has been studied in model systems containing purified sheep methaemoglobin (MetHb), nitrite and ascorbic acid. Evidence has been obtained that the conversion of ferric pigment to the nitric oxide form proceeds through the mediation of the unstable complex nitric oxide methaemoglobin (NOMetHb) rather than via deoxyhaemoglobin (Hb). A mechanism has been proposed which takes into account, and is in accord with, the findings of other workers in this field of study.