During the first 5 days after an intracranial haemorrhage, the red blood cells slowly haemolyse. Most of the oxyhaemoglobin, released in the cerebrospinal fluid, is transformed into bilirubin by an enzyme-dependent process. After 5 days, the haemolysis increases without a corresponding enhancement in the formation of bilirubin. Consequently, the oxyhaemoglobin concentration also increases. A spontaneous oxidation of the haem group follows and after about 10 days the proportions of oxy- and methaemoglobin are about the same. This process occurs independently of the cause of the haemorrhage.