The administration of methylene blue to assist the identification of insulin secreting pancreatic adenomata, during surgery in a patient with normal haemoglobin and red cells, was associated with an increase in the methaemoglobin concentration from 0.6% to 7.1%. In patients with unstable haemoglobins or abnormalities of the hexose monophosphate pathway the administration of large amounts of methylene blue is potentially dangerous.