Water and electrolyte balance was examined throughout the course of an experimental infection with T strain infectious bronchitis virus in colostomised chickens. Significant losses of water and negative sodium and potassium balances were observed. The major change in the electrolyte balance was the increased output of sodium in urine and this was associated with a diuresis. A decrease in food intake was the most important contribution to the negative potassium balance. Death resulted from acute renal failure. The implication of the results for electrolyte replacement therapy is discussed and a method for colostomies in birds weighing less than 0.5 kg is described.