1. Serial measurements of serum potasssium and total-body potassium were made on eighteen patients with megaloblastic anaemia before the start of therapy and during the period of recovery. 2. In those patients who presented with an initial packed cell volume of less than 25%, a mean decrease in serum potassium of 0.4 mmol/1 occurred on average with 2.5 days of the start of therapy. This was followed by a significant increase in serum potassium during the period of recovery. There was a significant increase in total-body potassium in these patients, but this could be explained by increases in erythrocyte mass, erythrocyte potassium concentration and lean body mass.