1. Protease activity of the mid-gut of 5th-stage larvae was studied under different conditions. The activity is not detectable in the unfed insect, but increases after feeding, reaching a maximum just before ecdysis.
2. In insects kept at high temperature immediately after feeding, protease activity is approximately normal for the first 2 days, but thereafter it decreases; after transfer to normal temperature the activity starts to increase.
3. Decapitation reduces protease activity under normal temperature conditions. Possible hormonal control of digestion is discussed, and it is suggested that the brain hormone is involved.
4. The concentration of free amino acids in the haemolymph greatly increases at high temperature, but returns to the normal level after transfer to normal temperature. The adjustment to the normal level does not occur if the insects are decapitated immediately after transfer.
5. The concentration of protein in the haemolymph at high temperature does not recover from the initial decrease. After transfer to normal temperature there is a tendency to a very slow increase.
6. Protein synthesis is impaired at high temperature and thus amino acids accumulate in the haemolymph, since they are not used in protein metabolism.
7. It is concluded that the impairment of protein synthesis results in the cessation of moulting, of cell division in the epidermis during wound healing, and of the secretion of the brain hormone.