1. The rate of urine secretion and the trans-wall potential have been measured in single, isolated Malpighian tubules in media of varying ionic composition.

2. Potassium, present as the only monovalent cation in a saline, is able to support urine secretion at a higher rate than sodium. In the absence of sodium, the rate of urine secretion depends upon the concentration of potassium, but is enhanced in the presence of sodium, especially at low potassium concentrations.

3. The replacement of part or all of the anions normally present in saline by sulphate results in a reduction in rate of urine secretion and a rise in trans-wall potential.

4. Neither the rate of urine secretion nor the trans-wall potential is affected by ouabain, but both are reduced by 2,4-dinitrophenol.

5. The ionic composition of the urine remains unchanged by the addition of diuretic hormone to the medium surrounding the tubule, but the rate of urine secretion and the trans-wall potential are increased.

6. It is suggested that the diuretic hormone acts by stimulating active potassium transport.

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