1. Contrary to expectation, it is found that metabolically useful substances pass into the urine and are reabsorbed in the rectum.
2. The kinetics of penetration have been investigated for six amino acids, three sugars and urea.
3. The evidence indicates that these substances enter the urine by passive diffusion.
4. The role of the Malpighian tubules in the insect's excretory system is discussed in the light of these findings. It is suggested that the tubule is primarily a means whereby all soluble substances of low molecular weight are removed from the haemolymph and that in this respect it has analogies with the glomerulus as well as with the tubule of the vertebrate nephron.