1. The excretion of sodium, potassium and water by the Malpighian tubules of the stick insect has been further studied in preparations of single tubules isolated in droplets of medium under liquid paraffin.
2. There is some gradation of physiological activity along the length of the superior tubule. Sodium, potassium and water are secreted into the tubule at all levels, but the sodium/potassium ratio is greater in the proximal region.
3. The proximal and middle regions of the inferior tubule have not been shown to differ in any way from the corresponding regions of the superior tubule. The distal dilatation has quite different properties and does not produce urine.
4. The rate of urine flow increases markedly as the potassium concentration in the medium is increased; the corresponding effect of sodium is barely detectable.
5. Sodium, like potassium, can be actively transported against an electrochemical gradient, and does not appear to compete with potassium in the secretory mechanism.
6. The rates of secretion of sodium and potassium vary in direct proportion to the respective concentrations of these ions in the medium. The rate of secretion of potassium is more than ten times greater than that of sodium.