The structural and functional characterization of haemoglobin (Hb) exceeds that of any other mammalian protein. Recently, the biological role attributed to Hb has been extended from the classical role in the transport and exchange of the respiratory gases O2 and CO2 to include a third gaseous molecule, nitric oxide (NO). It is postulated that Hb might be involved in the systemic transport and delivery of NO to tissues and in the facilitation of O2 release. However, definitive evidence for these putative activities is yet to be produced and many questions remain. Here we describe the present status of these hypotheses and their strengths and weaknesses.