Sixty patients who received massive blood transfusion intraoperatively and/or in the immediate post-operative period were analysed. Six patients had hypokalemia and two had hyperkalemia. The multifactorial changes leading to electrolyte disturbances especially involving potassium are discussed in relation to hypotension, hypothermia, acidosis, pH, and release of catecholamine. Potassium changes in relation to anaesthesia are discussed. The danger of routine administration of calcium during massive blood transfusion is stressed.