Fever regardless of its cause, increases the metabolic rate. This is because all chemical reactions in the body increase their rates of reaction, an average 12% for every 1°C rise in temperature. Elevated metabolic activity increases O2 demand, CO2 output and H2O and electrolyte requirements. The high-energy demand of elevated metabolism with fever imposes a severe strain on the vital organs of the febrile patient. Direct effects of fever (i.e. toxic effects) are depressant with loss of vascular tone and loss of capillary wall integrity resulting in relative hypovolaemia. Depression of vascular tone opens all the capillary beds and reduces the perfusion pressure to force blood through them. The injured capillary wall permits fluid loss. The net result is shock. The selection of fluid depends on whether the patient is having shock and/or high fever. Increasing tissue perfusion is vital in patients with shock and/or high fever. If treatment for fever has been started preoperatively, normal saline is to be started since there will be increased insensible loss due to sweating. This can be followed by Ringer’s lactate. If a patient comes to the operation theatre with history of treated fever, fluid resuscitation is mandatory before induction of anaesthesia. With normalization of temperature, there will be increased insensible fluid loss through sweat. In these patients, the loss through sweat can be excessive, which is from extracellular space.