A patient following head injury was admitted in intensive care ward with signs of raised intracranial pressure. He was put on a ventilator and started on intravenous fluids and diuretics. Twenty-four hours later his urine output was 3.5 litres, serum sodium 156 mEq/I and serum osmolarity of 316 mOsm/kg. The most likely diagnosis based on these parameters is -