1. The preparation of isolated Malpighian tubules is described. The rate of urine flow increases with increasing serum dilution, and vice versa. Urine is almost isotonic with haemolymph over a wide range of osmotic pressure.

2. Serum collected from 3-day-old insects promotes urine formation, whereas that from 6-day-old insects does not.

3. A factor which was extracted from the m.n.c. accelerates the rate of urine flow, from a normal value of 0·87 to 3·1 mm.3 x 10- 3/min. The osmotic pressure of the urine, however, remains unchanged.

4. The hormone concentration of different parts of the nervous system was assayed with these isolated tubules. Most activity occurs in the m.n.c., but some activity is present in extracts from the c.c. and the fused ganglionic mass in the mesothorax.

5. Malpighian tubules isolated from 6-day-old insects remain inactive, but after the addition of hormone they immediately begin to produce urine.

6. These observations have been incorporated into a tentative hypothesis on the control of excretion in Dysdercus.