The faecal output of chloride, sodium and potassium was studied in 14 patients with active idiopathic proctocolitis, in an attempt to establish a relationship between faecal electrolyte output and acid-base balance data. 7 patients with ileostomy and 8 healthy volunteers were used as controls. The daily faecal excretion of chloride and sodium in proctocolitis was significantly in excess of normal, whereas that of potassium was within the normal range. From the comparison with the results in patients with ileostomy it is concluded that colitic colon has an impaired capacity to absorb chloride and sodium but retains the ability to secrete potassium. The intestinal loss of chloride in addition to that of sodium and water may be regarded as a salt-losing diarrhoea and may account for the metabolic alkalosis commonly found in proctocolitis.