1. Single Malpighian tubules of the stick insect have been studied as preparations isolated in drops of haemolymph under liquid paraffin. Measurements of osmotic pressure have been made on haemolymph and urine.
2. The urine is more or less isotonic with the haemolymph over a wide (45-145%) range of osmotic pressure. But in a majority of cases the urine is slightly but significantly hypotonic to the haemolymph.
3. From these and other observations it is concluded that water must be actively transported across the wall of the tubule. The implications of this conclusion are discussed.
4. The normal rate of urine production is of the order of 0.07 x 1o-3 cu.mm./sq.mm./min. The rate increases with increasing dilution of the haemolymph and vice versa.
5. Urine production normally ceases, probably as a result of distension of the tubule, when the pressure in the lumen rises to about 20 cm. H2O.