1. The concentrations of sodium and potassium in the haemolymph and in the urine have been measured in eight species of insect.
2. The concentration of potassium in the urine is always greater than in the haemolymph. The concentration of sodium in the urine is generally less than in the haemolymph.
3. In seven of the species the difference of electrical potential across the wall of the tubule has also been measured.
4. In these seven species the results lead to the conclusion that potassium is actively secreted into the tubule. It is very probable that the same is true in the eighth case.
5. It seems likely that the excretion of sodium can be brought about by passive diffusion into the tubule.