The oxyhaemoglobin, methaemoglobin and bilirubin concentrations were determined in subdural haematomas and cerebrospinal fluid from 18 patients. The total haem derivative concentration ranged from 55 mumol/l to 13.9 mmol/l in the haematomas and from 0.1 mumol/l to 8.2 mumol/l in the cerebrospinal fluid. Bilirubin was the dominating fraction in haematomas with haem derivative concentrations lower than 1 mmol/l. For haematomas exceeding this value, the bilirubin transforming capacity seemed to have reached a maximum. More of the oxyhaemoglobin was oxidized to methaemoglobin in these cases, a reaction known to release superoxide radicals. The possible pathophysiological significance hereof, e.g. in cerebral vasospasm, is discussed.