1. This investigation has sought the site of release of the diuretic hormone which appears in the haemolymph of larvae of Rhodnius shortly after they begin feeding.
2. Material possessing diuretic activity can be extracted in high concentration from those lengths of the peripheral abdominal nerves which lie just behind the mesothoracic ganglionic mass.
3. An electron microscope study of the mesothoracic ganglionic mass and the abdominal nerves which leave it has shown that the axons from the neurosecretory cell bodies in the ganglionic mass some of which at least have previously been shown to contain the diuretic hormone run out into the proximal lengths of the abdominal nerves where they branch. Some of the branches have been followed to neurosecretory axon endings which typically are packed with neurosecretory granules and lie immediately beneath the fibrous nerve sheath.
4. By constricting fed insects at various positions it has been possible to show that most of the diuretic hormone is released into the region close to but behind the mesothoracic ganglionic mass.
5. It is concluded that the release of diuretic hormone in Rhodnius is from a series of swollen neurosecretory axon endings dotted over the surface of those lengths of the peripheral abdominal nerves which lie close to the mesothoracic ganglionic mass.