Blood samples of different carboxyhaemoglobin (CoHb) content (0%, 50%, 100%) were analysed, after short exposure to heat, for determination of the change of CoHb and methaemoglobin (MetHb) during the succeeding 14 days. Untreated blood showed an initial increase of MetHb after heating followed by a decrease during this period. There was no formation of CoHb in the untreated blood. When CoHb was already present in the blood the amount of MetHb was smaller in amount and was formed at a slower rate. In these samples the concentration of CoHb slowly decreased.