The sinus gland is a neurohemal organ located in the crayfish eyestalk and represents a storage site for neurohormones prior to their release into the circulation. The sinus gland contains 3 classes of dense, membrane-limited granules: 1) granules measuring less than 1000 Å in diameter, 2) granules measuring 1100-1400 Å in diameter, and 3) granules measuring 1500-2000 Å in diameter. Class 3 granules are the most electron-dense of the granules found in the sinus gland, while class 2 granules are the most abundant. Generally, all granules appear to undergo similar changes during release.Release of neurosecretory granules may be initiated by a preliminary fragmentation of the “parent granule” into smaller, less dense vesicles which measure about 350 Å in diameter (V, Figs. 1-3). A decrease in density of the granules prior to their fragmentation has been observed and may reflect a change in the chemical nature of the granule contents.