The effect of the i.v. administration of methylene blue dye on haemoglobin oxygen saturation, measured by a pulse oximeter was investigated. The absorption spectra of various concentrations of haemoglobin, oxyhaemoglobin and methylene blue were compared. The amount of 660-nm light absorbed by methylene blue was sufficient to cause a factitious haemoglobin desaturation as measured by the pulse oximeter.