1. The very considerable changes in the concentrations of sodium and potassium in the urine of Rhodnius during the normal course of excretion are not reflected in the haemolymph which remains relatively constant in composition.
2. The concentration of potassium in the haemolymph can be increased some tenfold by adding potassium chloride to the blood on which the insect feeds. Such increase in the concentration of potassium can be tolerated for several hours without loss of nervous co-ordination, but it is ultimately fatal.
3. The upper portion of the Malpighian tubule can secrete a urine which contains more potassium and less sodium than the haemolymph.
4. During the passage of this urine through the lower portion of the tubule the differences in concentration between urine and haemolymph became reduced.
5. It is probable that water and sodium, but not potassium, are reabsorbed from the urine in the rectum during the later stages of excretion.